314 ANNUAL OF SCIENTIFIC DISCOVERY. 



will permanently occupy the situation most remote from the earth. Let us 

 now consider what may be expected to be the distribution of air, water, or 

 other fluid on the surface of such a globe, supposing its quantity not suffi- 

 cient to cover and drown the whole mass. It will run towards the lowest 

 place, that is to say, not the nearest to the centre of figure or to the central 

 point of the mere space occupied by the moon, but to the centre of the mass, or 

 what is called in mechanics the centre of gravity. There will be formed 

 there an ocean, of more or less extent according to the quantity of fluid, 

 directly over the heavier nucleus, while the lighter portion of the solid mate- 

 rial will stand out as a continent on the opposite side. And the height above 

 the level of such ocean to which it will project will be greater, the greater 

 the excentricity of the centre of gravity. Suppose then that in the case of 

 the moon this excentricity should amount to some thirty or forty miles, such 

 would be the general elevation of the lunar land (or the portion turned 

 earthwards) above its ocean, that the whole of that portion of the moon 

 we see would in fact come to be regarded as a mountainous elevation above 

 the sea-level. In what regards its assumption of a definite level, air obeys 

 precise^ the same hydrostatical laws as water. The lunar atmosphere would 

 rest upon the lunar ocean, and form in its basin a lake of air, whose upper 

 portions at an altitude such as we are now contemplating, would be of exces- 

 sive tenuity, especially should the lunar provision of air be less abundant in 

 proportion than our own. It by no means follows, then, from the absence 

 of visible indications of water or air on this side of the moon, that the other 

 is equally destitute of them, and equally unfitted for maintaining animal 

 or vegetable life. Some slight approach to such a state-of things actually 

 obtains on the earth itself. Nearly all the land is collected in one of its hemi- 

 spheres, and much the larger portion of the sea in the opposite. There is 

 evidently an excess of heavy material vertically beneath the middle of the 

 Pacific; while not very remote from the point of the globe diametrically 

 opposite rises the great table-land of India, and the Himalaya chain, on the 

 summits of which the air has not more than a third of the density it has on 

 the sea-level, and from which animated existence is forever excluded." 



It has been very much the fashion, in works treating of the planets, to 

 take their actual distance from the sun, the regular formula for the diminu- 

 tion of radiated heat, and thence to construct theories which sound very 

 pretty in a lecture-room butter would be oil in Venus and a rock of ice in 

 Saturn but which will hardly bear philosophical investigation. 



Sir J. Herschel throws some light on this subject by hinting at the possi- 

 bility of different kinds of atmospheres. "All men do not dress in flannels. 

 It is very probable that a dense atmosphere surrounding a planet, while 

 allowing the access of solar heat to its surface, may oppose a powerful 

 obstacle to its escape, and that thus the feeble sunshine on a remote planet 

 may be retained and accumulated on its surface in the same way, and for the 

 same reason, that a very slight amount of sunshine, or even the dispersed 

 heat of a bright though clouded day, suffices to maintain the interior of a 

 closed greenhouse at a high temperature." Then, again, "the intensity 

 of gravity, or its efficacy in counteracting muscular power and repressing 

 animal activity on Jupiter is nearly two and a half times that on the earth, 

 on Mars not more than one half," and so on. " Lastly, the density of Saturn 

 hardly exceeds one-eighth of the earth's, so that it must consist of materials 

 not heavier on the average than dry fir-wood. Now, under the various com- 

 binations of elements so important to life as these, what immense diversity 



