Mb W. HOPKINS, ON THE EXTERNAL TEMPERATURE OF THE EARTH, &c. 629 



on the question, nor am I aware of any practical astronomer having entertained an opposite 

 opinion. Of Mercury we know too little by direct observation to form any opinion on these 

 points founded on observed facts, and the same remark will apply to the remoter planets 

 beyond Saturn ; but in the absence of any such direct testimony respecting these planets, 

 analogy would seem to throw the balance of probabilities far in favour of the opinion of the 

 existence of atmospheres in these cases as well as in that of the Earth, or in the cases of those 

 other planets which afford us more direct evidence on the subject. Most astronomers probably 

 feel much the same conviction that Mercury, Uranus, and Neptune have atmospheres of 

 greater or less extent, as that they revolve round their own axes with greater or less angular 

 velocity. 



It is not my object, however, to adjust the balance of probability for particular hypotheses 

 in favour of planetary atmospheres, or against them ; but, assuming their existence, to estimate 

 their effects on the planetary temperatures. And, in like manner, I shall point out the 

 influence which must be exercised by a greater or less conductivity and specific heat in the 

 superficial matter of a planet, without professing to discuss the probability of such properties 

 being materially different in the different planets. We are sure that there are powers in 

 Nature by which the great Author of Nature has adapted the conditions of each planet to the 

 purposes for which His wisdom has destined it ; and my object is to shew that, however limited 

 our knowledge of cosmical causes may be, we are still able to recognise certain natural 

 agencies by which the effects of heat derived immediately from the Sun and from stellar space 

 may be largely modified, and the resulting planetary temperatures adjusted to any purposes 

 for which each planet may be intended. 



2. Before I proceed to the particular points which the subject embraces, it may be well 

 to premise a few general remarks on the manner in which heat may be transmitted through 

 different media. This may take place (l) by complete radiation, (2) by conduction, (3) by 

 partial radiation and conduction, or (4) by convection. The transmission through vacuum, or 

 rather through space in which we conceive no other matter to exist than the thermaniferous 

 medium itself, is by complete radiation. Heat also passes through perfectly diathermanous 

 media in the same manner, precisely, as light passes through perfectly transparent media. In 

 such cases no portion of the incident heat is absorbed by the constituent particles of the 

 diathermanous medium, the temperature of which, consequently, remains entirely unaffected by 

 the transmission. This may be regarded as one extreme case ; the other is that in which heat 

 is transmitted entirely by conduction. In such cases the whole of the incident heat is absorbed 

 by the particles on which it first falls, and is then emitted by them to the next particles, and 

 so on successively till emission takes place from the opposite boundary of the medium. Such 

 media can only transmit heat when the temperature of the surface on which it is incident is 

 higher than that from which it is finally emitted. Again, the incident heat may be only partly 

 absorbed by the particles which form the external boundary of the medium, while other parts 

 penetrate to a greater or less depth within it before they are completely absorbed, precisely 

 as light penetrates to a certain depth in imperfectly transparent media. If this medium be of 

 sufficiently small thickness, a portion of the incident light will be transmitted entirely through 



