ON PNEUMATIC EQUILIBRIUM. 209 



brought to a certain level, indicated by a float, whatever portion of it may 

 be contained in the tube ; but the necessity of this adjustment may be 

 easily avoided, by allowing the mercury to play freely between two hori- 

 zontal surfaces of wood, of moderate extent, and at the distance of one 

 seventh of an inch : the height may then be always measured from the 

 upper surface, without sensible error. But if the surfaces were closer 

 than this, the mercury would stand too high in the tube. (Plate XIX. 

 Fig. 254.) 



The same method which is employed for determining the relation be- 

 tween the heights and densities of elastic fluids, may be extended to all 

 bodies which are in any degree compressible, and of which the elasticity is 

 subjected to laws similar to those which are discoverable in the air and 

 in other gases: and it is not improbable that these laws are generally 

 applicable to all bodies in nature, as far as their texture will allow them to 

 submit to the operation of pressure, without wholly losing their form. 

 Water, for example, has been observed by Canton* to be compressed one 

 twenty two thousandth of its bulk by a force equal to that of the pressure 

 of the atmosphere ; consequently this force may be represented by that of 

 a column of water 750 thousand feet in height ; the density of the water 

 at the bottom of a lake, or of the sea, will be increased by the pressure of 

 the superincumbent fluid ; and supposing the law of compression to 

 resemble that of the air, it may be inferred that at the depth of 100 miles, 

 its density would be doubled ; and that at 200 it would be quadrupled. 

 The same measures would also be applicable to the elasticity of mercury. 

 But there is reason to suppose that they are in both cases a little too 

 small. 



LECT. XXII. ADDITIONAL AUTHORITIES. 



Pascal, Nouvelles Experiences touchant la Vuide, 4to, 1647. Tables of the Com- 

 pression of Air under Water, Ph. Tr. vi. 2192, 2239. Sinclair, Ars Magna Gravi- 

 tatis et Levitatis, 4to, Rotterd. 1669. Mariotte, sur la Nature del' Air, 1676. Mari- 

 otte and Homberg on the Weight of Air. Hist, et Mem. de Paris, ii. 41. Homberg, 

 ii. 105. Wallis, Ph. Tr. 1685, p. 1002. Halley, ibid. 1686, p. 106. Derham, ibid. 

 1698, p. 2. Desaguliers, ibid. No. 386. Senguerd de Aeris Natura, 4to, Lond. 1699. 

 Cassini, Hist, et Mem. de Paris, 1705, p. 61. Lahire, ibid. 110, H. 10. Amontons, 

 ibid. 119, H. 10. Varignon, ibid. 1716, p. 107, H. 40. Forssell, Dissertatio Physica 

 de Barometro, 4to, Upsal, 1747. Scheuchzer, Ph. Tr. xxxv. 537, 577. De Luc, sur 

 les Modifications de T Atmosphere, 1 772. Shuckburgh, Observations made to ascertain 

 the Heights of Mountains by the Barometer, Ph. Tr. 1777, p. 513 ; 1778, p. 681. 

 Roy, ibid. 1777, p. 513. Playfair, Ed. Tr. i. 87. Dalton, Manch. Mem. v. Assier 

 Perricat, Nouveau Traite des Barometres, 1802. Lindenau, Tables Barometriques, 

 4to, Gotha, 1809. Biot. do. 1811. Ramond, sur la Formule Barometrique de la 

 Mecanique Celeste, 4to, 1811. Winckler, Tables Barometriques, 4to, Halle, 

 1820 and 1826. Carlini's do. Milan, 1823. Duvillard's do. Paris, 1826. Olt- 

 mann's do. Stuttg. 1830. Galbraith's do. Edinb. 1833. 



* Phil. Trans. 1762, p. 640 ; 1764, p. 261. See also Perkins, Ph. Tr. 1826, p. 

 561 ; and (Ersted's Report of the British Association for 1833 ; Trans, of Sections, 

 p. 353. 



