XIII. On the Measurement of Heights by the Barometer. In a To Mr. NICHOLSON. SIR, THE July 17th, 1809. barometer of HE method of finding heights by the barometer bids Finding fair to be of the greatest practical utility; especially since heights by the the improved construction of portable barometers, and the great practical invention of more compendious modes of calculation than utility. those formerly in use, have considerably diminished the difficulties, with which it was at first attended. have the calcu It would be desirable, however, if the necessary calcula- Desirable to tions could be still farther simplified: for it must frequently lations still farhappen, that observations of the heights of the barometer ther simplified. are made by travellers at times, when the mind, distracted by a variety of objects, or borne down by the fatigue of the body, may be ill calculated for even a moderate degree of exertion. For this purpose the following tables have been calcu lated, which, with little more than the mere trouble of inspection, will give the result true to the nearest foot. They may be printed on the surface of a common card, so that their bulk cannot be the least inconvenience to a traveller. Table 1. Contains altitudes in feet answering to every Use of the fol tenth of an inch of the height of the barometer from 25 to lowing tables. 31 inches. Table 2. Contains the proportional parts to be deducted for every additional hundredth of an inch, corresponding to the heights of the barometer marked in the first coJumn. Hence to find the approximate elevation of one station above another, nothing more is necessary, than to find from Tables 1 and 2 the elevations corresponding to the observed heights of the barometer, and subtract the less from the greater. It would not be difficult to construct tables, which should give the result by mere inspection: but unless they should be continued to every hundredth of an inch (in which case they would make a volume) the trouble of subtraction is all that would be saved. Table 3. Gives the correction for the expansion of air for every 1000 feet of altitude. It is calculated for every degree of the mean temperature from 72 to 32. It is probable that few observations will be made in this island, where the mean temperature is not within these limits. This table is calculated from Table 5, p. 484, of Gregory's Mechanics, vol. 1. With regard to the correction for the expansion of the mercury, it may be obtained without any sensible errour, by multiplying the difference of temperature in degrees of Fahrenheit by 2.75 feet, or 2 feet 9 inches. TH HE form of the glauberite is that of an oblique prism, Figure of the greatly depressed, and with a rhombic base. The angles of crystals of the parallelogram of the base are 76° and 104°. The angles glauberite. of incidence between the parallelogram of the base and the adjacent sides are 142°. That between the base and the edge contiguous to the acute angle of the base is 154°. The faces of the base are generally plane, smooth, and even shining: those of the sides on the contrary are full of striæ, parallel to the edges of the base. Very evident junctures parallel to the base are discoverable by cleaving; as are others not so well defined, parallel to the edges of the base, and inclined to the former in angles of about 104°. These observations give as the primitive form of this crys- Primitive form, tal an oblique prism with a rhombic base. The crystals are nearly limpid, or of a topaz yellow, and Colour. retain their solidity and transparency in the air, if they have not been wetted. * Journal de Physique, vol. LXVI, p. 235. VOL. XXIV-SEPT. 1809. Hardness. Action of fire. Singular action of water. Spec. grav. Crystals resembling it. Its component parts. No water. Sulphate of soda, and of lime. No loss. Their hardness exceeds that of sulphate of lime, but is in ferior to that of carbonate of lime. Exposed to the fire, the glauberite splits, decrepitates, and melts into a white enamel. Immersed in water, its surface becomes of a milky white, and in a little time the whole of the crystal grows completely white and opake. Taken out of the water and dried, it does not resume its transparency, but the white coating falls to powder; and, if it be entirely removed, the nucleus is discovered remaining unaltered. It is the only mineral substance that possesses this property. The specific gravity of the glauberite is 2.73. This salt, the crystals of which at first sight bear some resemblance to those of axinite, and the fragments of which are a little like those of sulphate of lime, differs essentially from the latter, whether anhydrous or possessing its water of crystallization, in its primitive form, and in the secondary forms derived from it. It is composed of anhydrous sulphate of lime ... 49 anhydrous sulphate of soda 51 .... 100 Mr. Brongniart satisfied himself, that it contained no water, not only by several calcinations at the temperature nearly of melting silver, but also by distilling it after Mr. Berthollet's manner with iron filings, when he could obtain no hidrogen gas. He ascertained the presence of the sulphate of soda by solution and crystallization, which afforded him well de fined crystals of this sulphate. The sulphate of lime he found by decomposing this salt both by carbonate of ammonia and oxalate of ammonia. As he had no loss, but what cannot be avoided in che◄ mical operations conducted with the greatest care, and this loss did not amount to one per cent, he presumes, that this stone contains no other ponderable matter essential to it but the two salts mentioned above: and to be more certain of this, he examined carefully, whether it contained no phosphates, phosphates, borates, or muriates, which might have been suspected from the situation where it was found. The glauberite was brought from Spain by Mr. Du- Where found. meril. It has hitherto been found only at Villarubia, near Ocanna, in new Castile. Its crystals are sometimes soli tary, sometimes in clusters, and disseminated in masses of sal gem. Mr. Brongniart has not been able to find any mention of this mineral, either in the works of minera logists, or in those of travellers in Spain, that he could consult. XV. An excellent colourless Copal Varnish. By Mr. LENOR- EVERY one knows the difficulty of dissolving copal com- Copal difficult pletely, when we attempt to make a varnish, I hasten there of solution. fore to communicate a method, that has succeeded perfectly with me; and which will be found, to produce a very fine varnish with this substance. All copal is not fit for making this varnish, it must there- Method of se fore be selected with care, and the following method will lecting it, show what is good. Take each piece of copal separately, and let fall on it a single drop of very pure essential oil of rosemary, not altered by keeping. Those pieces on which the oil makes a certain impression, that is to say, which soften at the part that imbibes the oil, are good, and should be reserved for making varnish. The others are to be rejected. Powder the pieces of copal thus selected, sift the powder and of making through a very fine hair sieve, and put it into a glass, on the the varnish. bottom of which it must not lie more than a finger's breadth thick. On it pour essence of rosemary to a similar height, stir the whole together with a stick for a few minutes, the copal will dissolve into a viscous substance, and the whole will form a very thick fluid. Let it stand for a couple of hours, after which pour on gently two or three drops of Sonnini's Bibliothèque Physico-économique for 1808, Vol. II, P. 193. F2 Tery |