634 Mk W. HOPKINS, ON THE EXTERNAL TEMPERATURE OF THE EARTH, 



Supposing, then, the Sun to be removed, the heat which the Earth would receive 

 from external sources would be that which radiates from all surrounding bodies in space, 

 of which we may regard as the most important the fixed stars. 



The diathermanous property of our atmosphere with reference to direct solar heat 

 has already been mentioned (Art. 2). We have no means of ascertaining by experiment 

 whether it possess the same property with regard to heat which radiates from the fixed 

 stars, but the analogy between the two cases is such that it would seem extremely difficult 

 to suppose this property not to exist with respect to stellar as well as solar radiating 

 heat. I shall here suppose it to do so equally. It is from this stellar radiation that 

 the temperature of general stellar space is derived, but what may be the amount of the 

 heat thus radiating to any particular spot, as our own planet, it is impossible to say, 

 because we can form no idea of what would be the temperature of stellar space if stellar 

 radiation were entirely to cease. The quantity of heat thus received by the earth may, 

 for ought we know, be much greater than that which we receive from solar radiation. 

 With respect to the uniformity of stellar radiation in different directions upon the Earth, 

 we can only assert that it must be nearly uniform from those regions of space towards 

 which the north and south poles of the Earth are almost permanently directed; for if it 

 were otherwise there would necessarily be a greater difference between the temperatures 

 of the northern and southern hemispheres of our globe than is shewn by observation to 

 exist. 



The same conclusion will not necessarily apply to the equatorial regions of the 

 heavens, since nearly the whole of the Earth's surface is presented to them in turn. 

 It will suffice, however, for our purpose, to suppose that stellar radiation upon the Earth, 

 or any other point within the solar system, is the same in every "direction, so as exactly 

 to resemble the radiation which I have supposed in our hypothetical cases, to proceed 

 from a uniformly heated spherical envelope of indefinite extent. 



8. In our hypothetical case I have supposed the heat to return through the shell 

 from the central nucleus entirely by conduction. There is no doubt, however, but that 

 all the modes of transmission heretofore specified (Art. 2), except that by complete 

 radiation, are efficient in the case of the Earth's atmosphere. The conclusion arrived 

 at above will still be equally true — the excess of t, above t will increase cceteris paribus, 

 with the extent of the atmosphere, though not in the same ratio as if the transmission 

 were effected by the slower process of conduction alone. The temperature t must now 

 be considered as that to which if the atmosphere were reduced radiation from it into 

 external space would entirely cease. We have no knowledge whatever of its value, except 

 that it must doubtless be exceedingly low. 



We may also remark that when radiation takes place between particles of the medium 

 within a certain limited range, the radiation into external space will not take place 

 altogether from those particles which form the extreme boundary, but from all those 

 situated within a certain distance of that boundary ; but this will not render a determinate 

 relation the less necessary between t 2 and t a . 



