42 Some Astronomical Problems. [Sess. 



the earth have afforded a host of speculative theories as to 

 their being the seat of life in any form, but more particularly 

 of reasonable beings. We have a good store of fairly well 

 ascertained facts relating to the planets, enough perhaps to 

 guide us to some just conclusion on this point. When the 

 genius of Copernicus moved the centre of our system from the 

 earth to the sun, our world became a planet, a heavenly body. 

 Now as we in our study of the fixed stars make great use of 

 the knowledge we have of our own sun to determine their 

 nature, so in the study of the planets and their physical con- 

 dition the intimate knowledge we have of our own earth may 

 give us very valuable help. I am inclined to believe that in 

 connection with this always fascinating problem of the habita- 

 bility of the planets, sufficient attention to the condition of the 

 earth as one of the planets has not been applied as fully as it 

 might have been. The earth, then, is a globe nearly 8000 

 miles in diameter, distant from the sun about 93 millions of 

 miles, possessing a fairly dense atmosphere, and with three- 

 quarters of its surface covered by water, the visible proportion 

 of land to water being 1 : 3. But when we consider the volume 

 of that water, then we find that the bulk of the land standing 

 above sea-level, when compared with the bulk of the water filling 

 the great ocean basins, the ratio becomes 1:14. Water has a 

 high specific heat, and on this account is able to absorb an 

 immense amount of heat radiated from the sun, and part with 

 it afterwards slowly. Our water envelope thus becomes a 

 vast accumulator and distributer of heat, while the atmosphere 

 enfolds the whole like a blanket, the two together forming a 

 great protection against the intense cold of space. In spite of 

 this safeguard, however, both poles of the earth are frozen ; 

 the line of eternal snow comes down to sea-level at a variable 

 point, due to seasonal changes, within the north and south 

 arctic circles. Sufficiently high mountains, even at the 

 equator, will partly pierce this protective covering of air, and 

 have their heads crowned with everlasting snow, the snow-line 

 in these regions reaching an altitude of 15,000 to 16,000 feet : 

 the barometric pressure is about 15 inches, indicating about 

 one half that experienced at sea-level. This is an important 

 point, as it shows how rapidly the reduction of atmospheric 

 density increases terrestrial radiation of heat, to such an 



