XXIX. On the External Temperature of the Earth, and the other Planets of the 

 Solar System. By W. Hopkins, Esq., M.A., of St Peter's College, F.R.S., and 

 Vice-President of the Society. 



[Read May 21, 1855.] 



1. Gkeat interest has recently been excited respecting the physical state of the different 

 planets which compose the Solar System, and more especially respecting those physical 

 conditions by which their surfaces may severally be characterised, and to which the consti- 

 tution of any organic beings existing on each particular planet must necessarily, as we conceive, 

 bear a direct and intimate relation. Of these external conditions, as we may term them, such 

 as heat, light, moisture, &c, I now propose to examine that of temperature. We have not, it 

 is true, sufficient data to determine the superficial temperature of any planet besides our own. 

 We know, however, that it must mainly depend on the temperature of the planetary space, 

 and on the direct heat which the nearer planets, at least, must receive directly from the Sun, 

 but modified, and possibly in a far greater degree than has been generally supposed, by the 

 particular circumstances by which each planet may be characterised. The modifying circum- 

 stances to which I shall more particularly direct attention in this paper, are the existence of 

 atmospheres surrounding the planets, the positions of their axes of rotation, and the con- 

 ductivity and specific heat of the substances forming the outer crust of each planetary body 

 of our system. No astronomer, I suppose, judging from the appearances which Mars and 

 Jupiter present to us, would entertain any serious doubt as to the existence of atmospheres 

 surrounding those planets, and the probability would seem to be almost equally strong of 

 Saturn being likewise enveloped in a similar manner. The obliquity of the axis of rotation is 

 known with considerable accuracy in the cases of Mars and Jupiter, and also in that of Saturn 

 if, at least, it coincide with the axis of rotation of his ring. Venus presents great difficulties 

 to the observer, on account of the dazzling quantity of light which she reflects to us, and the 

 absence of well-defined permanent dark spots on her disc. This led formerly to considerable 

 doubts as to the period of her revolution about her own axis, which appears now, however, to 

 be pretty satisfactorily determined to be nearly the same as that of the Earth. The position 

 of her axis presents a corresponding difficulty in its determination, but the most generally 

 received opinion is, I believe, that the obliquity is large, amounting, in the opinion of some 

 astronomers, to as much as about 75°. This must produce an extraordinary difference between 

 the changes of annual temperature in that planet, and those which we experience. I have 

 endeavoured, in this paper, to estimate numerically the effect of this anomalous obliquity. 

 The indications of an atmosphere about Venus are not perhaps so distinct as those in Mars 

 and Jupiter, but still the opinion of its existence is sanctioned by such observed facts as bear 



