ought to have lost weight, whereas it was found in | either fixed in the case immediately under the needle many cases that the results of combustion were heavier and separate from it, or it is attached to the needle than before combustion had taken place. The dis- itself, as in Fig. 611, so as to traverse with it. In covery of oxygen gas proved fatal to the phlogiston the former case it is made of cardboard or metal; in theory. Lavoisier burnt phosphorus in a jar of the latter, of some very light substance not liable to oxygen, and observed that much of the gas disap- warp from heat or moisture, such as a thin plate of peared, and that the phosphorus gained in weight; talc. that the increase of the one was in the ratio of the decrease of the other. Iron wire burnt in oxygen gas gave a result equal to the wire employed, plus the weight of the oxygen that had disappeared. Mercury being confined in a vessel of oxygen, and exposed to the temperature of about 600°, the gas combined with the metal, and the resulting oxide being heated to about 900°, was reconverted into oxygen gas and metallic mercury; the quantity of oxygen thus recovered answering precisely to that employed in the first instance to produce the oxidation.

In ordinary cases of combustion the heat evolved does not depend upon the combustible, but upon the quantity of oxygen that enters into combination. Thus, according to Despretz, a pound of oxygen, in combining respectively with hydrogen, charcoal, alcohol, &c., evolved in each case nearly the same amount of heat, each raising 29 lbs. of water from 32° to 212°. A given weight of different combustibles gives the comparative quantities of heat represented by the following figures :

In the magnetic compass, the plane of the horizontal circle is divided into 32 parts by lines supposed to be drawn diametrically through the circle. These, as practically applied to the compass card, are called points of the compass, or in nautical language, rhumbs. In marking the compass card Fig. 610, the circle is first divided into two semicircles by a diameter NS, which denotes the line of the magnetic meridian; and

W

MSM

WNW

MS

NW

NNW

N

NNE

NE

baked wood. . . .

36

ether

236

,, hydrogen The products of combustion in oxygen or its compounds form the three great classes of acids, alkalies, and orides.

COMPASS, an instrument consisting of a magnetic needle or bar, mounted on a fine centre, enclosed within a shallow box or metallic case, and furnished with a plane circular card, denoting the chief or cardinal points of the horizontal plane about us. The term compass is immediately derived from the card which compasses or involves as it were the whole plane of the horizon. The compass-needle NS, Fig. 609, is usually a light

bar, set edgewise upon an agate centre; sometimes it consists of a thin piece of steel plate tapering from the centre to the extremities, and it may be of any dimensions according to the size of the compass required. The compass or card which indicates the various points in the horizon with reference to the direction of the magnetic needle, is

Fig. 609.

the north point, as being the most elementary or great point of reference, is usually distinguished by an ornamental arrow or fleur-de-lis. A diameter E w is next drawn at right angles to the other, by which we obtain the east and west line, and thus we have the 4 principal or elementary cardinal points. The quadrants of the circle between these 4 points are further and equally divided by 2 other diameters producing four new rhumbs or points. These are named from their relative position in the compass. The point midway between N and E, for example, being compounded as it were of the two directions, is termed north-east, and is marked N E. That midway between N and w is termed north-west, and marked NW: that between s and w is termed south-west, and marked sw: that between s and E is south-east, and marked SE.

We thus obtain 8 principal points or rhumbs, and by continuing the division by diameters, bisecting the arcs contained by these first 8 points, we obtain an additional 8 points, making in all 16 points: these additional points are named as before from their position in the compass. The point midway between N and NE is termed north-north-east, as being nearer north than east, and is hence marked with two letters N, thus NNE. In a similar way the point between east and north-east is termed east-north-east,

as being nearer the east, and is marked thus, ENE; | points, as referred to the magnetic meridian of the and so on of the remaining bisected arcs: thus we have the points NNW and WNW for the points between north and west; ssw and wsw for the points between south and west; SSE and ESE for the points between south and east.

By continuing to bisect the arcs included between these points, we again double the number of rhumbs, and obtain 16 additional points, making in all 32 points: these are also named from their position in the compass, with the addition of the word by. Thus the point midway between N and NNE is called north by east and is marked N by E; that between N and NN W, north by west, and is marked N by w; the point between NE and NNE is called northeast by north, and is marked NE by N, and so on, leaning for the designation towards the nearest of the four elementary cardinal points. Thus the point midway between E and ENE is termed east by north and is marked E by N. In this way we arrive at 32 rhumbs or divisions of the circle into points, which taken in succession from the first or principal point, north, and carried round the circle in either direction, east or west,-suppose in the east direction, will stand thus:

particular locality, are correctly placed. The land compass is also occasionally employed in the measurement of angles in surveying instruments. It may be, for general purposes, of any moderate size, from that of a common seal up to the diameter of a foot. The land compass has usually a spring stop under the needle, by which it may be thrown up and retained clear of the point when not in use.

A fixed compass-card, as applied in the land compass, could not be used on ship-board for determining the position of the cardinal points in reference to the magnetic meridian, because the vessel is continually varying its position, and is in continual motion: it becomes therefore requisite to construct the compass-card of some light substance, not liable to warp or to be injured by heat and moisture. This is attached to the needle itself, so as to admit of both card and needle traversing together. If the needle be fixed to the card with the north and south poles immediately under the north and south poles of the compass, then all the points of the card will be correctly placed in reference to the magnetic meridian of the place, in whatever direction the vessel be turned; that is to say, supposing that no disturbing influence from iron or other cause exist in the ship itself. This is therefore the principal distinction between the land and sea compass. In the sea, or mariner's compass, Fig. 611, the magnetic needle,

An enumeration of these successive points from memory is called boxing the compass. More minute divisions of the compass card are estimated by what are called half and quarter points, each point being divided or supposed to be divided into 4 equal parts, so that any small angular quantity between either of the 32 divisions or points just enumerated, as, for example, between N and N by E would be termed, north-a-quarter-east, or north-half-east, or threequarters-east, as the case may be: we then arrive at north by east, and so of all the other points. Thus north by west, a little to the north or west, would be called north by west a quarter, or half, &c. north, or a quarter, or half, &c. west, as the case may be.

For more refined purposes, the compass is enclosed by a graduated circle, divided in the usual way into 360 degrees, by which the rhumbs are estimated in angular quantities, each rhumb or point being the d part of 360°, or 11° 15': a half-point will be then 5° 37' 30"; a quarter-point 2° 48′ 45".

When the compass-card is fixed to the box or case in which it is enclosed, and the needle is allowed to traverse over it, we have what is usually termed a land compass. It is commonly used by travellers for determining the different points of the horizon, the box being turned so as to bring the north and south points of the card immediately under the north and south poles of the needle. In this case, all the other

Fig. 611. THE MARINER'S COMPASS. together with its card, s w N E, is accurately poised on a fine central point within a bowl or case, A, of glass, metal, or wood; and in order to prevent any disturbance from the pitching and rolling of the ship, this bowl is set within a ring of metal, CA D, upon two axial pivots, projecting from its opposite sides, one of which is seen near A. The ring, in its turn, is set also upon axial pivots, at c and D, in a line at right angles to the former, and which are supported either within a second semicircular or vertical ring, CB D, or on pivot notches in two brass plates fixed at C and D to the box or case in which the whole is usually enclosed. [See BINNACLE.] The centre of gravity of the mass is frequently kept far below these axial pivots of suspension by a ring or small mass of lead, attached to the bottom of the compass-bowl. The two brass circles within which the compass-bow! is thus supported are called gimbalds; and it is evident that, by these, the card and needle will gene rally be preserved in a plane perpendicular to that of

COMPASSES-CONCENTRIC-CONE,

423

double pair of compasses, two legs being above and two below the central joint: the simplest form of these compasses is that called wholes and halves, because the longer legs being twice the length of the shorter, when the former are opened to any

the point of suspension. In fact, any rolling motion | For copying and reducing drawings, compasses of transverse to the axis CD moves the ring CAD upon a peculiar construction are used, consisting of a the axis c D, and any similar motion transverse to the axis a moves the ring CAD upon the inner axis A; the interior bowl, with the needle and card, being all the while maintained in a vertical position by the force of gravity. This form of the compass, although chiefly employed to guide the mariner across a track-given line, the shorter ones will be opened to the half less ocean, has still many other important practical applications. For an account of these, we refer the reader to Sir W. Snow Harris's Treatise on Magnetism, published in Weale's "Rudimentary Series," in which he will find an account of the azimuth compass, the dipping needle, and other magnetic instruments.

of that line. In this way, all the lines of a drawing may be reduced to one-half, or enlarged to double their length. To reduce or enlarge drawings in any required proportion, a beautiful instrument, named the proportional compasses, is used. This, together with beam compasses, will be described under PANTAGRAPH. For further information respecting compasses, we must refer to Mr. Heather's Treatise, in Weale's "Rudimentary Series." This is the best and cheapest treatise on the use of mathematical instruments with which we are acquainted.

CONCENTRIC, having the same centre: concentric circles are those described about the same

In a right

I

B

с Fig. 612.

COMPASSES, an instrument in its simplest form consisting of two legs, movable about a joint. The extremities of the legs are furnished with points, which may be set at any required distance from one another, and thus be used to measure and transfer distances, and to describe arcs and circles. "The points of the compasses should be formed of well-point. tempered steel, that cannot be easily bent or blunted; CONCRETE. See LIME-MORTAR. the upper part being formed of brass or silver. The joint is framed of two substances; one side being of the same material as the upper part of the compasses, either brass or silver, and the other of steel. This arrangement diminishes the wear of the parts, and promotes uniformity in their motion. If this uniformity be wanting, it is extremely difficult to set the compasses at any desired distance, for, being opened or closed by the pressure of the finger, if the joint be not good, they will move by fits and starts, and either stop short of, or go beyond, the distance required; but when they move evenly, the pressure may be regulated so as to open the legs to the desired extent, and the joint should be stiff enough to hold them in this position, and not to permit them to deviate from it in consequence of the small amount of pressure which is inseparable from their use. When greater accuracy in the set of the compasses is required than can be effected by the joint alone, we have recourse to the hair compasses, in which the upper part of one of the steel points is formed into a bent spring, which, being fastened at one extremity to the leg of the compasses, almost close up to the joint, is held at the other end by a screw. A groove is formed in the shank, which receives the spring when screwed up tight; and by turning the screw backwards, the steel point may be gradually allowed to be pulled backwards by the spring, and may again be gradually pulled forwards by the screw being turned forwards."-Heather. There are also compasses with movable points; the end of one of the shanks being formed with a spring, which holds firmly the movable point, or a pencil or ink-point, or a pen-point with a dotting-wheel, or a length-surface of a right cone is found by multiplying the ening-bar, as may be required. To describe small arcs or circles, a small pair of compasses, called bow compasses, with a permanent ink- or pencilpoint, are used. They are formed with a round head, which rolls with ease between the fingers.

CONE. If a straight line A x, Fig. 612, pass through a fixed point o, and be moved through any curve such as A B C, it will trace by its motion a surface, which is called a cone. Of this cone, the point o is the vertex; the right line, the motion of which produced the surface, is the directrix, and the curve ABC which guides its motion, the generatrix of the cone. If the generatrix be a right lined figure, the cone will become a pyramid. The most usual form of cone is that whose generatrix is a circle. The axis of a cone is a line drawn from its vertex to the centre of its circular base. cone, the axis is perpendicular to its base; in an oblique cone, the axis is oblique to its base. A pyramid and a cone, whose bases and altitudes are equal, have equal volumes; for since all the corresponding sections parallel to the bases are equal, the cone will be composed of a series of plates, equal respectively to those which compose the pyramid. The volume of a cone is found by multiplying the area of its base by one third of its altitude. If a cone and a cylinder have equal bases and equal altitudes, the volume of the cone will be one third of the volume of the cylinder. The volumes of cones being proportional to the products of their bases and altitudes, and the bases being proportional to the squares of their diameters, the volumes will be proportional to their altitudes multiplied by the squares of the diameters of their bases. The area of the

length of its side, by half the circumference of its base. The area of the surface of a right cone, is equal to that of a triangle, whose base is equal to the circumference of the base of the cone, and whose altitude is equal to the side of the cone.

If a cone

be cut by a plane A'B', Fig. 613, parallel to its staves across the wood must be wrought into segbase, the figure with parallel circular ments of a circle, and be thickest nearer the middle, bases thus cut off, is called a truncated growing gradually thinner towards the ends. When come. The area of the surface and the the staves are dressed and arranged in a circular volume of a truncated cone, are found, form, the cooper without attempting to slope their, by taking the difference of the surfaces, so that the whole surface of the edge may touch in and of the volumes of the whole cone, every point, brings the contiguous staves into conBAO B, and of the cone A'o B' which is, tact only at the inner surface, and by driving the cut off. hoops hard, he can make a closer joint than could be done by sloping them from the outer to the inner sides. In this, and in giving the proper curve to the staves, consists the principal part of the cooper's art.