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tions or ordinates are placed at the rabbets of the stem anil post, as before explained; and the distance between them, measured on.the middle line of the half-breadth plan, is divided into a number of equal portions suitable for the application of the rules given for obtaining curvilinear areas, and at the points of division lines are drawn perpendicular to the middle line. These lines represent transverse sections perpendicular to the upper edge of the rabbet of the keel, and in Fig. 19 are represented by the vertical sections 1, 1J, 2, &c. The ordinates are then measured off from the half-breadth plan, and inserted in the table, as already described in our former example, and/the calculat:ons proceeded* with as though the transverse sections were perpendicular to the load water sections.

In our present example, the distance between the extreme .vertical sections measured on the middle line is divided into 18 equal portions, each being 9-4 feet; and for greater accuracy, the two extreme intervals are again divided into two equal portions respectively, as is shown in the Fig. 19.

In placing the horizontal sections they are so arranged that the third rule for approximating the curvilinear areas may be applied. There are at the upper portion of the displacement four equidistant sections, and at the lower portion four other equidistant sections. The common distance between the sections L. W. L. and 2 W. L.; 2 W. L. and 3 W. L.; 3 W. L. and 4 W. L., is 2-65 feet, measured on the sections drawn perpendicular to the rabbet of the keel, and the common distance between the sections 4 W. L. and 4J W. L; 4J W. L. and 5 W. L.;

6 W. L. and 5J W. L. is feet, measured on

2 the same sections.

Performing the calculations in the same manner as described iu the former example, we obtain first the areas of the projections of the several horizontal sections; and before being multiplied by j rd of the common interval they are to be found in the first horizontal column below the ordinates; they are 3275,93-75,194-75,297-5,452-8. 550-95, and 621*4. Suppose we have multiplied these results respectively by one-third of the common interval, and so obtained the areas of the projections of the leveral horizontal sections, these areas will be equal to the true areas of the respective sections, multiplied by the fraction -=., L being

Jj

the distance between any two vertical sections, measured on a horizontal section, and I the distance between them measured on the middle line of the half-breadth plan; the true area of any horizontal section may therefore be readily found from these areas given in our table. Generally,

however, the fraction - is so near unity that the

areas found in the table may be taken as the true areas of the sections, without any practical error; and it is only in cages in which the difference of draught of water forward and aft is very considerable in proportion to the length of the ship, that

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the multiplication by the fraction will be necessary.

Let us assume, however, that we have obtained the true areas of the horizontal sections, and let them be respectively represented by Alt As, A3, ^4, A^, Ay and A^.

To obtain the displacement from these we have only to multiply them respectively by 2, 6, 6, 3, 3, 3, 3, and 1, in accordance with the third rule for finding a curvilinear area, and their sum multiplied by Hhs of the perpendicular distance between the sections, will give the displacement. Now, the perpendicular distance between any two sections is equal to the distance between them, measured on a vertical section, multiplied by the

fraction -: it is obvious, therefore, that if we L

make use of the projections of the areas of the horizontal sections (which are the true areas multiplied by the fraction 1) and take the distances

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between the horizontal sections, as measured on a vertical section, we shall obtain the correct displacement.

To obtain the displacement by transverse sections, the rationale of the operations will be readily made out. For, since the common interval between the horizontal sections is measured along the transverse sections in the sheer plan, when the ordinates for any section are put into the rule, and the sum multiplied by the proper fraction of the common interval, the result will be the true area of that section. And since the common interval between the transverse sections is the perpendicular distance between any two consecutive ones; when their areas are put into the rule, end the sum multiplied by the usual fraction of the common interval, the result will evidently be the displacement.

Another explanation may be given of the manner of finding the displacement inclosed by the extreme sections from the projections of the outside of the planking, as delineated on the building draught. Taking, as before, the extreme transverse sections at the outside of the rabbet of the stem and stem-post at the load water line, let the distance between them be measured on the L. \V. L. in the sheer plan, and then divided into the same number of equal parts as was the distance between them on the middle line of the half-breadth plan, and through the points of division let transverse sections be drawn perpendicular to the upper edge of the rabbet of the keel; these sections will, of course, coincide with those before drawn, but since the common interval is measured on the horizontal sections, the trne areas of the horizontal sections will be found on the table, and not their projections as before. Again, the common interval between the horizontal sections must now be measured perpendicularly to them, and not along the transverse sections, as in the former case; it is easy to see, therefore, that the true displacement will in the end be arrived at by the horizontal section*.

To proceed with the transverse sections we shall

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first have their projections on a plane perpendicular to the load water line, and not their trne areaj, but as before the true displacement will be obtained when these are put into the rule,*»nd the common interval taken as measured along the horizontal sections.

With these observations, we shall leave the student to work out the example for himself, as no difficulty can present itself respecting that portion of the displacement which is inclosed by the extreme sections; the work is precisely the same as in our former example.

The small portions of the displacement situated outside of the extreme sections are fonnd by the ordinary rules of mensuration, as also art' their centres of gravity, and the necessary corrections are made in the results before obtained.

In the present example the portion below 5J W. Jj. contains 167 cubic feet, and its centre of gravity, sitnated at i, Fig. 19, is 120 feet from section 1, and 12-5 feet from the L. W. L.

The contents of the rudder and stern ports abaft the section 19 are 65 cubic feet, and the centre of gravity situated at c, Fig. 19, is 180 feet abaft section 1, and 6-25 feet below L. W. L.

The portion between the stem-and the extreme sections, which must be subtracted, contains 22'8 cubic feet, and its centre of gravity, situated at a, Fig. 19, is 2'5 feet abaft section 1, and 9-5 feet below L. W. L.

The positive portions amount to 167 + 65, or 232 cubic feet. The negative portion is 22-8; the total positive appendage is therefore ,209-2 cubic feet, which must be added to 28198'-2757, the displacement sitnated between the extreme sections, to obtain the total displacement. Ib forming the scale of displacement, the proper appendages must also be taken into the calculations:

Again, in obtaining the centre of buoyancy in our former example, there were no appendages to the displacement. In the present case, the centre of buoyancy corresponding to the portion inclosed by the extreme sections is found as in that elample, and it then remains to find the effect of the appendages upon its position in a vertical and also in a horizontal direction. This may be retktilr done by means of the property of the centre of gravity proved in our last article, page 215. Assume that the axes o x, o y both pass through the centre of gravity, above found, of the displacement inclosed by the extreme sections, and that they are parallel to L. W. L. and section 1, then we have only to find the moments of the appendage) about ox and oy.

On reference to the table it will be found that the centre of buoyancy without the appendage* is 4-3115 feet from the L. W. L. and 86-69 fot from section 1; consequently the centre of pravitj of the keel piece is 12-5—4-3115, or 8-1885 feet from the L. W. L.; therefore 167 x 8-1885 = 1367 5 is the product of its weight and distance from or, also 6-25—43115 = 1-9385 is the distance of the centre of gravity of the rudder and post* from ox; and 9-5—4-3115 = 5-1885 i* the distance of the negative portion forward from the sami «xu,

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therefore 65x 1*9385=1260, and 228 x 5-1885= 118'3; the sum of the positive products is 1493*5, and the negative product 118*3, the result will therefore be positive, and equal to 1375-2. This quantity divided by 28407, the total displacement in cubic feet, including appendages, will give •0485 feet for the distance of the centre of buoyancy below that corresponding to the displacement between the extreme sections, or the centre of buoyancy below L. W. L. is 4-36 feet. In a similar manner, the distance that the centre of buoyancy moves aft by the addition of the appendages may be found to be 34 feet, as given in the table. The centre of buoyancy is therefore 87-03 feet abaft section 1, when the appeudages are taken into account.

The student will now, we think, be able to make the ordinary calculations for any ship, with either of the usual horizontal planes for the plane of projection, and in the above example he will complete for himself the drawing containing the acale of displacement, the scale of the area of the midship section, and that of the tons per such immersion similar to those given on page 298, voL I., for our first example.

Monsteb Tuoop Stiamkk.—The following paragraph has been communicated:—Messrs. Pearse and Co., Stockton, have on the stocks for the Government an Indian river steamer of immense proportions. Her length in the water-line is 350 feet, over all 375, and breadth 46 feet. She is by far the largest river steamer in the world. Her engines are 200 horse power. She wHJ be impelled by paddles, and will, it is expected, attain a Bpeed of 13 miles an hour. She will be fitted with sleeping berths and every suitable sanitary arrangement for about 800 soldiers. She will be guided by two large patent steering blades, and U made to draw only two feet of water, even with all her stores, fuel, and 800 passengers on board. She is flat-bottomed of course, and weighs about 370 tons. She will be completed in about a month, and will then be fitted up on the Thames, in order to make an experimental trip.

CAPTAIN COLES' PATENT SHIELDS FOR

ORDNANCE. Captain Cowpbb Phipps Couss, R.N., has recently patented an apparatus for defending guns and gunners in ships of war, gunboats, and land batteries. His invention consists of a large convex shield covered all over its exposed portion with thick iron or other metal, and mounted upon a platform or frame which is capable of revolving after the manner of a turntable, and which also carries the gun upon any suitable carriage. An aperture is formed in the shield to allow the muzzle of the gun to pass through it, and this aperture is somewhat larger vertically than horizontally in order to admit of the elevation and depression of the gun, the lateral aim being secured by the rotation of the platform. Captain Coles does not limit himself to any precise form or mode of construction, but he usually prefers to make the shield hemispherical, and to construct it of a wooden frame with timbers in vertical planes placed close together, the whole being covered with thick iron plates. In the engravings hereunto annexed, we have illustrated the manner in which he carries his invention into effect. The figure is a transverse section of a shield, constructed according to his invention, and adapted to a land fort or battery. The body of the shield is composed of a compact framing of timber A, the whole exposed upper portion of which is covered with thick iron plates B. The shield is built upon a revolving timber platform or frame C, which turns about a central bolt or stud Z>, and runs upon conical rollers E, placed beneath it near its circumference. These rollers are carried by arms F, which are attached to a loose running central ring G. The conical rollers E, run upon -i metallic rail or tram, supported by a Boor or platform If. The rotary motion which is requisite in the training of the gun is obtained by means of a hand-wheel I, fitted within the shield and connected with gearing, which works a pinion K, that travels round in teeth or cogs formed upon the circumference of the metallic rail or tram. L is the aperture through which the gun protrudes.

The aperture L is only sufficiently wide to allow the muzzle of the gun to pass through it, but is of sufficient size vertically to allow of the depression and elevation of the piece. A narrow aperture I is continued up from the main aperture L, to admit of the gunner " sighting" the gun with ease and accuracy. When the shield is formed and fitted as shown in the engraving, the ammunition must be supplied through an aperture made for the purpose in the rear of the shield. But in some cases, as on board ship for example, Captain Coles prefers to employ a hollow cylindrical casing in place of the solid central bolt or stud D, and pass the ammunition up from below through it. In like manner he proposes in other cases to modify other details of the apparatus. Instead of making the shield of an arched or curved form, for instance, he makes it polygonal in horizontal section, and straight-sided in vertical section, or of any other prismatic or other form, which will present a convex or inclined surface throughout to shot and shelL He likewise sometime* mounts the shield upon spherical balls, or rollers, or otherwise, instead of upon the conical rollers before described, and in place of the hand wheel and gearing for rotating it, he employs any other suitable appliances for the purpose. Further, he provides for the escape of smoke from the shield, and for its ventilation, by forming apertures in it for the purpose, and sometimes (as where a central upward passage is provided for the ammunition) by forcing air continually through the shield by means of a fan placed beneath it. Finally, he proposes to form the shield large enough in some cases to cover two or even more than two guns side by side and parallel to each other. When the shield is employed on board ship it should be either built or placed somewhat eccentrically in order that thecommon centre of gravity of the shield and the gun or guns beneath it may be caused to fall in the centre, about which the whole rotates. This will prevent the same from having any disposition to rotate imparted to it by gravity as the ship rolls or pitches. Captain Coles prefers to mount the gun9 placed beneath the shield upon. carriages resembling that invented by Jlr. Marshall in 1827, but modified to suit the purpose more effectually. We. have no account of the manner in which this modification of the carriage is to be effected, and have consequently had engraved within the shield a gun and carriage taken from Commander Marshall's original description of 1827. The following account of this part of the engraving is derived from the same source. "A is the breech carriage,* which differs only from the ordinary carriage in its shape before, which will be understood sufficiently by the drawing; B is the coin; C is the trunnion, and 1) the asp over the trunnion; K, la the muzzle carriage, having its wheel or wheels F, in this position at right angles with the wheels of the breech carriage j E a mere solid block of wood; G, H are two plates bolted to it with eyes at one end r and /, to receive the hinged bolt J; this bolt passes through another eye .?, which U bolted to the ship'n sides and steps into a socket made for it in the water ways; K represents the ship's side, and L the water ways, and » the socket; N is a strong iron crutch, with a piece of hard wood i, i, fitted in the middle of it for the muzzle of the gun to traversn upon; this crutch turns on the central bolt or pivot P, which is set into a socket made for it in the block or bed of the muzzle carriage A'; the gun is hire supposed to be run out in a straight direction for firing, and if it were fired the recoil would cause the gun to run in the inuz/.le, and traversing on the wood i, i, till the crutch came to the muzzle rim, or beyond it if required according to the lengths of the ropes or breeching*. By this arrangement a hand spike or tackle applied to the muzzle carriage at -3T, on either side, will point the gun with the greatest nicety and facility, the wheel or wheels F being, as will be observed, in the proper position for that movement." It should bo observed that the characteristic feature of Commander Marshall's carriage consisted in mounting the breech and muzzle of a gun on two separate carriages, one supporting the breech and the other the muzzle, and the wheels of one being at an angle with those of the other.

DOUBLE CYLINDER EXPANSION MARINE

ENGINES.

At but year's meeting of the British Association, Mr. John Elder, Engineer, of Glasgow, gave a description, with plans, of the various double cylinder expansion marine eugines for ocean steamships that had been constructed by his firm during the last six years. At the late Aberdeen meeting he added those of the Callao, Lima, and Bogota, together with those of some others now in progress. Before entering on this subject he reminded the Association that these engines are constructed with the view of getting the greatest amount of power from a given quantity of steam, at a given pressure, with less total weight of engine-boilers and water, and occupying less total space than that occupied by the ordinary class of steam-engines on board of steam-ships. At the former meeting he had mentioned that the above steam-ships, with the ordinary first class system of machinery, were coming from Valparaiso to this country to havo their engines replaced by these double-cylinder engines, entirely for the purpose of saving of fuel—the Lima and Bogota. They are of the following dimensions:—Length, li)5 feet; breadth, 36 feet; depth, 23 feet; breadth between paddle-boxes, 86 feet; tonnage, 1,630 tons. These ships had formerly a full poop and forecastle, which were altered this year into a spar deck, and the hulls of the ship increased in weight about 120 tons; the cabin and hold accommodation was lengthened 30 feet by the space saved by the unaller quantity of coal and boiler space required for the new machinery; and the registered tonnage of the ship was increased by upwards of 400 tons. The following statements are in the language of Mr. Elder. In the engines of the Lima, Callao, and Bogota,

• We here preserve Mr. Marshall's language, bii(viUiout reproducing the letters of reference in the engraving.

the cylinder capacity is so great as to admit of the steam being expanded to within two pounds of the pressure in the condenser at the end of the stroke, while the engines are working full power; and as the nominal power of these engines is contracted to be 320, the cylinders arc 90 in. diameter, and 5 feet stroke; and, in order to reduce the violent shock of high-pressure steam on such a large piston, and also to increase the jacket surfaces, a cylinder of one-third the size, or 25in. diameter, and 5 feet stroke, is placed close to it. This small cylinder receives the steam direct from the boiler during one-third of its stroke; this steam is consequently reduced to one-third of its original pressure at the end of its stroke, and then enters the second cylinder, where it is expanded three times more, making a total of nine volumes. Thus •12 Ibs. steam is expanded nine times, or to 4$ Ibs., namely, from 42 Ibs. to 14 Ibs. in the small cylinder; it then enters the lartje cylinder at 14 Ibs., and is expanded to 41 Ibs., but as the second piston is three times the size of the first, the gross load will be the same on both pistons, and the piston rods, crossheads, and connecting rods may be duplicates of each other.

From the above pressures of steam at the entering of the cylinder, it is evident, that unless the inside surface of the large cylinder is retained about 210 degrees, condensation of the steam on entering is certain, and such condensation will chiefly evaporate into the condenser while the eduction port is ouen, and the latent heat necessary to evaporate such condensation will be much greater than what would bave radiated from the hot cylinder to the condenser, had no condensation taken place ; and such heat would be entirely lost. In the same manner it might be mentioned, that the inside surface of the small cylinder should be retained a.s high in temperature as the steam that enters it; and in order to attain this object every effort should be made in the construction of steam machinery. It is evident that, for the small cylinder, superheated steam is absolutely necessary for this purpose, either in the jacket or cisterns of the cylinder; and in the large cylinder the temperature of steam direct from the boiler to the cylinder may be sufficient, if communicated through a large enough pipe or aperture.

In the engines under description the pipe supplying steam to the jackets was 24 in. diameter, and the steam was superheated to upwards of 100 degrees that entered the jacket. It was found that a large supply to the jacket saved a vast quantity of heat, which can only be explained by the principles above mentioned, namely, that any quantity of heat supplied to the jackets assisted in proportion to the quantity of latent heat it saved being evaporated to the condenser during the eduction of the steam, and if the pipes to the jackets were large enough, or sufficient to prevent the condensation referred to, the economy of the machinery was realized to the greatest extent.

Mr. Elder called the attention of all parties concorned to the performance of Cornish pumping engines, and more particularly to the similarity of action of the steam jacket in these engines to the principle of that of the double cylinder engine with steam jackets. In the Cornish engine the piston it is single acting, and the jacket has twice the time to do its work, or rather the steam in the cylinder is twice the time in contact with the jackets that it is generally with Watt's engine, also that the Cornish engines have very large jacket surfaces in proportion to the power developed. With these features in view, the engineers constructed the engines now under discussion, and to this cause may be attributed a considerable portion of their success, and to the non-observance of these features the almost total failure of economy in the expansive working of most steam engines on board of steamships, namely, by constructing large engines, going slow, without steam jackets, or superheating of steam; such engines would, of course, present a most favourable opportunity for improvement by adding any mode of superheating apparatus.

In the Cunard line, and many of our ocean steamers where very high steam chests and large

hot uptakes pass up through the steam, the economy of expansion has never been questioned, but can be substantiated to a considerable extent at any time. It is likely, however, that were the temperature of the steam from such boilers ascertained, it would be found considerably superheated. And the use of the various modes of superheating has been rendered much more neceiaary than it otherwise would have been, by engineer! generally following that type of tubular boiler used ly the Admiralty where there are low steam chests, and short uptakes, said to be rendered necessary in the case of the Admiralty on account of exposure to shot.

From the foregoing it is also conclusive that with the ordinary construction of steam-engine* afloat, that small engines going fast would consume less coal per indicated horse-power thin large engines going slow; but with engines such as those of the Callao, Bogota, and Lima the converse will be the case, carried, of coune, to within moderate limits.

Again, to return to the case of the Callac, Lima, and Bogota, the general con^tructwn of the eugines and boilers are as follows :—The cylinder* are four in number, namely, two of 62 inches diameter, and two of 90 inches diameter «nd 5 feet stroke. The cylinders lie diagonally to each other. The steam and eduction valves are wrought with eccentrics, the steam valve i» a gridiron valve with a large lap; one eduction valve Kms as eduction for the hi^h and its comspondia; lowspressed cylinder; it bus no lap, and the eduction port remains open during tfle entire stroke of the piston, thereby giving a free egrew for the steam, aiid ample escape for water «ho"ld it form.

In reversing the engines the eccaqtrici ire made to overrun the engine* by a donkey-engine till they arrive at the backing position—« p)«n which is less likely to cause accident than-the ordinary methods. "This donkey-engine has leeii found to be most satisfactory in its application.

The boilers are tubular, two in number, with iron tubes. Each boiler has three furnaces, 3 feet 4 inches wide, and 6.4 feet long, or making an aggregate of 130 square feet of fire-grate. Tin; tubes are of iron, 283 in number, 4 inches wait diameter, and 6 j feet long. Each vessel has 13 oval steam chest, 12 feet high, and 8 feet lon^, and 6 feet broad, with three uptakes through thi: steam chest each 2 feet diameter and 15 feet long.. This makes a strong form of take-up where it joins the tube plate, especially in boilers firing acr«> the ship; the feed-pipe of the boilers enters inUi a long flat touk or shield in front of the furnac;in which the furnace doors are fopned. Til* shield forms a protection to the firemen fr«" heat, and makes the heat, otherwise lost, aviil able for the feed-water. In the Callao theni» a third coil of feed-pipe in the funnel, ti/ heat the feed-water. Such, then, are the lea&n; features of this machinery, and the results are a* follows:—

This plan of the boilers gave steam to the «• gines superheated to about 400 degrees by tl>' uptakes, showing that the various systems _u! superheating are unnecessarily complicated; indeed, in the Lima, the steam got so far above i<X> degrees, that in the Bogota the steam chests wm' made two feet lower, and two small feed-pi])1' were made to feed the boiler when too much superheated by the tap of the steam chest. The auyorheated steam, though upwards of 400 degrees K heat, was found quite inadequate to prevent condensation in the cylinder, without the stcain-ja^ct cock being full open.

Mr. Elder pressed this fact on notice, as in tw case of double-cylinder engines it is so prominent!' observed, by comparing the respective diagr»!<i< of the low and high-pressed cylinders, especial'.1 as those engines, where the cylinders are to c!o« that the diagram of one is an exact counterpart of the other, .when there is no condensation: and it w somewhat curious to observe, while taViu-.' diagrams of the low pressure cylinder, ^'e gradual development of the diagram, with tu» jacket-cock full open, compared with that when it is shut.

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When the steam was at a pressure of 21 pounds shore the atmosphere the temperature at the iurface of the water was 264 dees., and at the top of the steam chest 400 degs. Faht., showing that the steam was surcharged to the extent of 13G degs., notwithstanding thiit the steam was in direct and unimpeded contact with the surface of the water. The engines made during the trial trips, which were generally half-a-day in length, from about 23 to 26 revolutions, and indicated from 1,000 to 1,300 horse-power during that time, and consumed from 20 to 25 cwts, per hour, with the surface blow-off cocks open. The 1 '.i11. i.. Lima, and Bogota have all shown a consumption of from 2 to 2} pounds per indicated horse-power per hour best Welsh coals, and the speed of the ships from 124 to 13 knots per hour. The steam-ship Callao has now been plying between Valparaiso and Panama with Her Majesty's mails for upwards of nine months, and has performed her work in a most satisfactory manner. The distance between these is upwards of 3,200 miles, and this she performs regularly on about 30O tons of coals. The Callao made the run from Liverpool to Valparaiso in, I think, about 36 days (teaming time, which averages about 210 miles per day during a run of 9,000 miles on a consumption of about 20 cwts. per hour. The Lima has also arrived at her destination after a most successful run; she performed the distance of 1,500 miles, from Valparaiso to Callao, in 141 hours, conxurnine 150 tons of coals, logging at an average of 260 miles per day during that distance, considerably faster than she had ever done with her original engines, and on less than half the coals consumed. The Bogota was completed and tested on the first of September last, and found fully eqnal to the others. She made the run from the Cloch Light in the Clyde to the Hell Buoy at Liverpool in 15 hours, against a strong head wind, and consumed during that distance 15 tons of Scotch coals.

At the Admiralty trial, which took place immediately on her arrival at Liverpool, she averaged upwards of 13 knots, the engine made 25^ revolt tions; she indicated I'OSO horse-power, and consumed about 21 cwts. per hour of Scotch coals; the steam was superheated to 340 degrees on entering the cylinder, and the thermometer at the water level of the boiler showed 264; the pressure in the boilers was 27 Ibs., and the vacuum in the condensers 26 inches. She left Liverpool for Valparaiso on the llth of the present month, with sufficient coals to carry her 5,000 miles at 240 miles per day, and a full complement of stores for the passengers on board; her draught of water on leaving Liverpool at the load line was, aft, 14 ft. 6 in.; forward, 13 ft. 9 in.; and displacement, 1,700 tons. She steamed to the Holyhead Light, where the pilot left her, at the rate of 11] nantical miles per hour against a strong head wind; the engines were making 20 revolutions; the steam pressure was 26 Ibs.; the vacuum 26 inches; and the consumption of coals 22 cwt*. best Welsh coals per hour.

The engineers are now constructing the ma chinery for three other steam-ships on this principle, with boilers on the cellular cylindrical spiral principle. In conclusion, the form of engines now described gives regularity of motion, while working expansively to the fullest extent, the expansion principle is fully realised, and the engines are of a strong architectural figure, with the various (arts easily got at, and reduced to simple forms, and present every facility for reversing freely by the engine-driver.

The performance of the Lima on a trial trip ol considerable distance, namely, between Liverpool and Dublin, was made several months ago, under the observation of a member of the Steamship Performance Committee, appointed by the British Association at its last meeting at Leeds.

The member watched the performance of the ship and her machinery with the utmost minute nt», in conjunction with the Admiralty engineer and an accurate return has been made to the committee, in accordance with the form prescribec and recommended for general adoption; and ii

will be found on reference to the Report of the Committee on Steamship Performance, presented n the Association a few days ago, that all the

dimensions and particulars of performance of the

'lima are embodied therein. The recorded performances of the Limn was not mere measured mile trial, which may always be

made to appear more favourable than practical

experience afterwards attains, but was such an 'Xtended trial as the committee had previously

recommended, and which recommendation was

adopted by the Directors of the Pacific Steamship

Company, the owners of the Lima.

SILAS'S MARINE LIGHTS, tfa. Febdinand Silas, inventor of the indestruc;ible lights applicable for telegraphic purposes at sea, and also to point out the presence of life-buoysin case of shipwrecks or other such disasters, and which were exhibited for the approval of the Lords of ,he Admiralty in September last, in the presence of Commodore the Hon. Sir James Drummond, '.B., and the officers of Woolwich Dockyard, last night performed a course of experiments at Blackwall for the satisfaction of Admiral M'Hardy, Capt. F. R. Ward, members of the Royal National Lifeboat Institution Committee; Capt. Robertson, Surveyor-General; Mr. Lewis, Secretary; Mr. 3rey, and other members of the Board of Trade who were present. The experiments commenced at six o'clock, when a number of the floating lights, attached to lines, were cast into the river from the pier. On floating to the surface, the phosphoric substance of which they are composed suddenly burst into a blaze of light, and was only extinguished by being dragged under the water. The apparatus intended for the telegraphic light' resembles a moderator table-lamp. This was placed on the jetty near the water's edge, and was operated upon by the inventor. In spite of the opaque fog, which completely enshrouded the river from bank to bank, the strong glare of the phosphoric light penetrated for some considerable distance, and was gratefully acknowledged by several boatmen, who would otherwise have had great difficulty in making the pier. Similar experiments were entered into a few days ago in the presence of Admiral Gordon and the Brethern of Trinity House, together with the officers of the exploring ship Fox. It was then pronounced that the invention was extremely suitable for all the purposes -above name:!, but they could not recommend the apparatus as applicable for lighthouses. In consequence of the deep obscurity in which the river was last night involved, and the difficulty of thoroughly carrying out the experiments, it was arranged, at the request of Admiral Hardy, that a second course of experiments shall take place at Blackwall on some future occasion.—Times.

STEAM SHIP ECONOMY.

TO TUB EDITORS OP TUB "MECHANICS'MAflAZINE."

Gkntlbmen,—Mr. Cheverton, I believe, admits the applicability of the formula based on the midship section (A); it therefore appears to me that he cannot object to my making use of this same formula expressed in terms of the displacement (DS) unless he is prepared to question the geometrical fact that the corresponding sectional areas of similar bodies vary as the square of the cube root of the cubical contents, that is, when applied to shipping the midship sectional areas of the immersed bodies vary as the square of the cube root of the displacements. As many of your readers are now taking an interest in the theoretical performances of large ships in the reasonable hope that the Great Eastern will afford the opportunity of practically testing the value of such theoretical calculations, and as the essay (Steam Ship Capability, 2nd edition) to which I made reference in my letter of the 1st instant, being now out of print, is not readily accessible, and as the promotion of public discussion, with a view to public utility, is the only object that I have in publication, you would do me a favour, and perhaps some of your correspondents an acceptable service, by inserting

in the Mechanics' Magazine that portion of my essay which relates to the superior capabilities of large as compared with smaller vessels; and I take the liberty of enclosing a copy of this paper in the hope that you will publish it in the Mechanics' Magazine at your earliest convenience. As to Mr. Cheverton having understood that I am not the original discoverer of the formula referred to, I presume that he got that information from my own writings.

I am, Gentlemen, yours very obediently,

Chables Athebtojt. Woolwich Dockyard, 14th November, 1859.

SHIPS' CHAIN CABLES.

M T >, Nov. 10,1859.

TO THB EDITORS OP THE " MECHANICS' MAGAZINE."

Gentlemen,—Your correspondent "Amiens," in connection with the destruction of the Royal Charter, has done much good, by enlightening the public upon the character of the iron plates used in the construction of iron vessels. There i» one point, however, upon which his exposition is imperfect. I refer to the quality of the ironused in the chain cables, and I pray a short space for a few lines on that subject.

The first misfortune £hat we read of in this sad relation of the destruction of the Royal Charteris that after the anchors were cast the two cables sn apped, The readers of the graphic article in the Times, "The Great Eastern in the gale," will not require to have their memories refreshed how the cables did duty in her case. We read—" In this hut struggle some of the links of the chain cable were actually dragged out one-third longer, and one, which passed under the sharp bows of the vessel, was bent nearly double."

It is the writer's opinion had the Royal Charter cables been of the same quality as those of the Great; Eastern that vessel and her cargo of precious metal and more precious lives would not have been lost. I have heard it stated and repeated that the proprietors of Urge vessels care not anything further about the quality of their cables, than thai they stand the test required by the insurance1 offices. A very common or inferior iron cable will stand that test, when new. Yet such a strain may be received then, although the cable may not have snapped, that it would have been far safer to have used the cable without testing it. But with a superior quality of tenacious fibrous iron, no injury whatever is done to the cable by the test employed, and it would not be torn asunder without almost donble the weight employed. I have spoken myself to cable manufacturers upon the impropriety of using common No. 2 bolts for making cables, and have received some such answer as that they received no complaints, and used the iron of the best makers. I* there no one to complain for the host of lives, as valuable as our own, which have been lost in the last fortnight's storms from this cause? I know nothing of the cables of the Royal Charter, but cannot help forming an unfavourable opinion of them, and suppose them to be no better than the plates employed.

The cables of the Great Eastern, as has been stated, were made by Messrs. Brown, Lenox, and Co., Cardiff, and of iron prepard for them at the works of Mr. Anthony Hill (Hill P F Co.) j and the late Mr. Brunei had most emphatically requested Mr. Hill, that the lust possible iron only should be provided,—and probably better iron was never seen than that used in the cables of the Great Eastern, and it was not a bit too good, as the proof as shown.

Allow me to close this subject in your own words. "This state of things should be remedied. In the name of Christian charity, let us not expose our fellow-creatures to violent deaths, 500 at a time, for the mere difference in the price per ton between good and bad iron." "VEBHAa."

TO THE EDITORS OP THE "MECHANICS' MAGAZINE."

Gentlemen,—I feel assured you will pardon me for bringing the attention of the public through your columns to the apparent ultimate cause of the loss of the noble steamer the Royal Charter

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