Mr. J. Croll on the Theory of Gravitation. 449 



Atmospheric Changes which produce Wind and Rain/' called 

 attention to the great effect which condensation must produce; 

 but he attributes that effect to the further expansion of the air 

 by tbe latent heat made manifest, rather than to the actual loss 

 of weight, or elastic force, in the atmospheric column. This 

 view has also been partially adopted in the later editions of the 

 1 Physical Geography of the Sea/ by Captain Maury, who con- 

 siders that the latent heat developed by condensation may give 

 a further expansion to the tropical air. It seems to me, on the 

 contrary, more probable that the effect is produced by the direct 

 statical agency of diminished pressure, and that the heat evolved 

 is simply thrown off into space through the very thin and clear 

 medium of the upper atmosphere. 



Whether this is the case, or not, it is difficult (perhaps impos- 

 sible) to decide ; but I submit that I have shown strong grounds 

 for believing that, in whatever way it acts, sudden condensation 

 of aqueous vapour is the principal cause of the trade-winds, of 

 their inflection towards the west coast of Africa, and of the 

 Indian monsoons, in opposition to the theory which would de- 

 rive these winds from the mere expansion of the intertropical 

 air by heat. 



LIX. On certain Hypothetical Elements in the Theory of Gra- 

 vitation and generally received conceptions regarding the Con- 

 stitution of Matter. By James Croll*. 



Gravitation. 



NO future researches or discoveries in physical science will 

 ever overturn Newton's grand theory of universal gravita- 

 tion, or ever in the least degree shake universal confidence in its 

 truth. It will stand as the immoveable foundation upon which 

 the whole superstructure of physical science will for ever rest. 

 Although the truth of everything that is really essential to the 

 theory is established beyond the possibility of a doubt, yet there 

 are certain hypothetical elements which have been unnecessarily 

 associated with it, or rather included in it, which by no means 

 can lay claim to be considered as established. I shall briefly 

 refer to a few of these elements. 



Gravity is commonly defined to be an "attractive force between 

 the particles of matter varying inversely as the square of the 

 distance." Or, as stated more fully, " every particle of matter 

 in the universe attracts every other particle with a force varying 

 inversely as the square of their mutual distances, and directly 

 as the mass of the attracting particles." It will be seen at once 

 * Communicated by the Author. 



