OR UNIVERSAL DICTIONARY OF THE ARTS, SCIENCES, LITERATURE, &c. INTENDED TO SUPERSEDE THE USE OF OTHER BOOKS OF REFERENCE. ILLUSTRATED WITH THREE HUNDRED AND SEVENTY PLATES AND MAPS. SECOND EDITION, IN TWENTY-THREE VOLUMES. VOLUME XVIII. EDINBURGH: PRINTED BY JOHN BROWN, ANCHOR CLOSE, FOR THE PROPRIETORS, AND SOLD BY ALL THE BOOKSELLERS IN THE UNITED KINGDOM. 1816. E56 18 ENCYCLOPÆDIA PERTHENSIS. PNEUMATICS. (Concluded from Volume Seventeenth.) ANOTHER eafy method is this: Let an apparatus abcdef, pl. CCLXXXI. fig. 48.) be made, confifting of a horizontal tube ae of even bore, a ball dge of a large diameter, and a fwan-neck tube bf. Let the ball and part of the tube geb be filled with mercury, fo that the tube may be in the fame horizontal plane with the furface de of the mercury in the ball. Then feal up the end a, and connest ƒwith an air-pump. When the air is abftracted from the furface de, the air in a b will expand into a larger bulk ac, and the mercury in the pump gage will rife to some distance below the barometric height. This diftance, without farther calculation, will be the measure of the elafticity of the air preffing on the furface de, and therefore of the air in a c. The most exact method is to fufpend in the receiver of an air-pumpa glass vessel, having a very narrow mouth over a ciftern of mercury, and then abftract the air till the gage rifes to fome determined height. The difference e between this and the barometric height determines the elafticity of the air in the receiver and in the fufpended veffel. Now lower down the veffel by the flipwire till its mouth is immerfed into the mercury, and admit the air into the receiver; it will prefs the mercury into the little veffel. Lower it ftill farther down, till the mercury within it is level with that without; then ftop its mouth, take it out and weigh the mercury, and let its weight be . Subtract this weight from the weight of the mercury, which would completely fill the whole veffel; then the natural bulk of the air will beau, while its bulk, when the elafticity e in the rarefied receiver, was the bulk or capacity w of the veffel. Its denfity therefore correfponding to this elafticity e, was fs be obtained. Various experiments for this purpose have been made. Thofe made by M. de Luc, General Roy, Mr Trembley, and Sir George Shuckbourgh, are by far the most accurate; but they are all confined to very moderate rarefactions. The general refult has been, that the elasticity of rarefied air is very nearly proportional to its denfity. No regular deviation from this law has been obferved, there being as many observations on one fide as on the other; but it is certainly worthy the attention of philofophers to determine it with precision in the cafes of extreme rarefaction, where the irregularities are most remarkable. The great source of error is a certain adhesive sluggishness of the mercury when the impelling forces are very small ; and other fluids can hardly be used; because they either fmear the infide of the tube and diminish its capacity, or they are converted into vapour, which alters the law of elasticity. Upon the whole, we may affume the Boylean law, viz. that the elafticity of the air is proportional to its denfity. The law deviates not in any fenfible degree from the truth in those cafes which are of the greatest practical importance, that is, when the denfity does not fo much exceed or fall fhort of that of ordinary air. With respect to the action of the particles on each other, the inveftigation is extremely easy. We have feen that a force 8 times greater than the preffure of the atmosphere will compress common air into the 8th part of its common bulk, and give it 8 times its common denfity: and in this cafe the particles are at half their former distance, and the number which are now acting on the furface of the pifton employed to comprefs them is quadruple of the number which act on it when it is of the common denfity. Therefore, when this eightfold compreffing force is diftributed over a fourfold number of particles, the portion of it which acts on each is double. In like manner, A when |