204 LECTURE XXII. 



4to, Paris 1744. Cotes's Hydrostatical and Pneumatical Lectures, 1747. Euler 

 on Hydrostatics, &c. Hist, et Mem. de Berlin, 1755, p. 217, &c. Nov. Com. 

 Petr. xiii. xiv. xv. Lecchi, Idrostatica ed Idraulica, Milan, 1765. Kastner, An- 

 fangsgriinde der Hydrodynamik, Gott. 1769. Bossut, Traite d'Hydrodynamique, 

 2vols. 1777. Lambert on the Constitution of Fluids. Hist, et Mem. de Berlin, 

 1784, p. 299. Bernard, Hydraulique, 4to, Paris, 1787. Langsdorfs, Theorie der 

 Hydrodynamischen Grundlehren, Frankf. 1787. Hydraulik, 4to, Altenb. 1794. 

 Parkinson's Hydrostatics, 4to, 1789. Burja, Grundlehren der Hydrostatik, 1790. 

 Eytelwein's Handbuch der Mechanik und Hydraulik, Berlin, 1801 ; translated by 

 Nicholson. Mollet Hydraulique Physique, Paris, 1810. Raccolta di Autori Ita- 

 liani che Trattono del Moto dell' Acque, 19 vols. 4to, Bologna, 1821-4. Gauss, 

 Principia Generalia Theorise, fig. Fluid, in Statu yEquilib. 4to, Gott. 1830. 



Elementary Treatises will be found in many of the works on mechanics already 

 mentioned, besides which are the following : Francoeur, Paris. Vince, Camb. 1812. 

 Bland, Camb. 1824. Moseley, Camb. ; Miller, Camb. 1831. Webster, Camb. 

 Moreau, 4to, Brest, 1830 ; together with the treatises in Brewster's Cyclopaedia, 

 &c. &c. 



LECTURE XXII. 



ON PNEUMATIC EQUILIBRIUM. 



THE laws of the pressure and equilibrium of liquids, which are the 

 peculiar subjects of hydrostatics, are also applicable in general to fluids of 

 all kinds, as far as they are compatible with the compressibility of those 

 fluids, or with their tendency to expand. 



Elastic fluids are distinguished from liquids by the absence of all cohesive 

 force, or by their immediate tendency to expand when they are at liberty. 

 Such are atmospheric air, steam, and gases of various kinds ; and the consi- 

 deration of these fluids, in the state of rest, constitutes the doctrine of pneu- 

 matostatics, or of the equilibrium of elastic fluids. 



That the air is a material substance, capable of resisting pressure, is easily 

 shown by inverting an empty jar in water, and by the operation of trans- 

 ferring airs and gases from vessel to vessel, in the pneumatic apparatus 

 used by chemists. The tendency of the air to expand is shown by the 

 experiment in which a flaccid bladder becomes distended, and shrivelled 

 fruit recovers its full size, as soon as the external pressure is removed from 

 it, by the operation of the air pump : and the magnitude of this expansive 

 force is more distinctly seen, when a portion of air is inclosed in a glass 

 vessel, together with some mercury, in which the mouth of a tube is 

 immersed, while the other end is open, and without the vessel ; so that 

 when the whole apparatus is inclosed in a very long jar, and the air of the 

 jar is exhausted, the column of mercury becomes the measure of the expan- 

 sive force of the air. (Plate XIX. Fig. 251.) 



If the diameter of the tube, in an apparatus of this kind, were very 

 small in comparison with the bulk of the air confined, the column of mer- 

 cury would be raised, in the ordinary circumstances of the atmosphere, 

 to the height of nearly 30 inches. But supposing the magnitude of the 



