84 DISCUSSIONS IN CLIMATOLOGY. 



different effects produced by water falling as a liquid 

 in the form of rain and as a solid in the form of snow. 

 The rain, however much of it may fall, runs off 

 rapidly, he states, without producing any permanent 

 effect on temperature. But if snow falls, it lies where 

 it fell, and becomes compacted into a mass which 

 keeps the earth below and the air above at or near 

 the freezing-point. When the snow becomes per- 

 petual, as on the summits of high mountains, 

 permanent cold is the result; and however strong 

 the sun's rays may be, the temperature of both the 

 air and the earth cannot possibly rise much above 

 the freezing-point. "This," he says, "is illustrated 

 by the often-quoted fact that at 80° N. lat. Captain 

 Scoresby had the pitch melted on the one side of his 

 ship by the heat of the sun, while water was freezing 

 on the other side owing to the coldness of the air." 

 Doubtless this is perfectly correct; but on page 502 

 he states that he has pointed out with more precision 

 than has, he believes, hitherto been done, the different 

 effects on climate of water in the liquid and solid 

 states. This is a somewhat doubtful statement ; for 

 in Chapter IV. 'Climate and Time,' in "Phil. Mag." 

 March, 1870, and in other places will, I think, be 

 found all that this section contains. In fact the 

 influence of snow and ice as a permanent source of 

 cold is one of the main factors of my theory. The 

 three great factors are (1) the influence of snow and 

 ice, (2) the influence of aqueous vapour, and (3) the 

 influence of ocean-currents. How persistently has it 

 been urged as an objection to my theory that, during 

 the glacial epoch, the great heat of the perihelion 

 summer would more than counterbalance the effect of 

 the aphelion winter. But I have maintained that the 

 summers, notwithstanding the intensity of the sun's 



