the Structure of the Earth. 67 



It follows thailess heat or repulsive energy would be necessary 

 to attain the limit of terrestrial elastic fluidity, while more was 

 required to reach the confines of the solid and tenacious liquid 

 states, at a great elevation, as upon the summit of a lofty moun- 

 tain, than at the mean of the earth's surface, or (which is not 

 very much lower,) the level of the sea. The difference, however, 

 in requisite quantity, is not very considerable at any accessible 

 elevation. It amounts to but a thousandth part at the height of 

 nearly two miles above the level of the ocean, and two such parts 

 at the height of four. 



To pursue this remark, it may be noticed, that the effect of 

 gravity is augmented or diminished by the co-operation or op- 

 position of lunar and solar attraction. For it is obvious, that, 

 if both those forces be acting on the same line and direction 

 with that of the earth's gravity, whether in retaining the particles 

 of a liquid, or in changing their relative positions, a greater 

 power will be required to balance or overcome its effect, than 

 when the same forces are opposed to the gravitation of the 

 earth's mass. The difference, however, is too inconsiderable to 

 be made a subject of observation, as the difference between the 

 concurrence of lunar and solar attraction with terrestrial, and 

 their opposition to it, scarcely amounts to a millionth part. 



It is not, however, needless to take that ihto account, in con- 

 sidering the subject. For, when a gas has reached its limit, 

 if it do so, where repulsion of its particles might precisely equal 

 their mutual attraction, joined with their external gravity, should 

 the power of terrestrial gravity be then counteracted, and its 

 effect diminished, by the contrary attraction of the moon, the 

 elastic fluid might no longer be restrained by the earth's gravi- 

 tation, but become a free elastic fluid, confined by no terrestrial 

 forces, but only retained in its place by repulsion of contiguous 

 particles of the like ethereal fluid universally diffused. 



By gravitation, a particle of vapour, which has barely passed 

 the liquid limit, is restrained from abandoning its position rela- 

 tively to the earth's mass, and is made to hover at the surface ; 

 and by its excess of repulsion above attraction of contiguous 

 particles, it presses, as it is pressed upon, with like or with 

 greater excess of repulsion above attraction. 



