576 , PHILOSOPHICAL TRANSACTIONS. [aNNO 1779- 



air = 1000 when the English thermometer of Fahrenheit is at Si", if the heat 

 of the air be increased '12.8 degrees of this tliermometer, its volume will be, 



According to Mr. Abbe De La Caille 1040 



Mr. Professor Mayer 1046 



Mr. Bonne 1047-7 



Sir George Shuckburgh 1050.5 



Dr. Bradley 1054.4 



'■ Here then are great differences in this point only, namely, the effect of heat 

 on the density of the air; differences which must have a visible effect in all me- 

 teorological phenomena. Doubtless they proceed, in a great measure, from the 

 different degrees of dryness of the air, which we can no longer doubt of, since 

 the interesting experiments of Col. Roy with the manometer. This is the same 

 cause to which I imputed the greatest variations in my experiments in open air, 

 and which obliged me to conclude my correction for the effects of heat on the 

 air from a mean among my numerous observations. Now this mean, reduced 

 to the same term of comparison as above, is 1047, which is also a mean among 

 those different results. 



XXX 11. On the Precession of the Equinoxes produced by the Snn's Attraction. 

 By the Rev. Mr. Isaac Milner, M. A., and Fellow of Queens College, Cam- 

 bridge, p. 505. 



If the actions of the sun and moon on the different parts of the earth were 

 equal; or if the earth itself were perfectly spherical, and of a uniform density 

 from the centre to the surface, in either case the attractions of those remote 

 bodies would have no effect on the position of the terrestrial equator, and the 

 equinoctial points would constantly be the same in the heavens. But it was im- 

 possible to give the earth a rotatory motion round an axis without giving at the 

 same time a centrifugal force to its parts. This force is greatest at the equator, 

 and is in a contrary direction to that of gravity; on either side of the equator the 

 force is less ; and besides, only part of its effects is opposed to that of gravity. 

 It is usual, in determining the figure of the earth, to consider the whole mass 

 as in a state of fluidity, and the different columns as sustaining each other at 

 the centre. If the earth be considered as a hard body, firmly cohering in its 

 parts by some other force besides that of gravity, it does not seem necessary that 

 the different columns should be supposed to sustain each other at the centre, 

 though in both cases the direction of gravity must at every point of the surface 

 be perpendicular to the tangent of the figure. But we know that there is a con- 

 siderable quantity of water on the surfiice of the earth ; and therefore, if the 

 equatorial regions were not higher than the polar, they certainly would be over- 

 flowed by the ocean, which is contrary to experience; and for this reason the 



