REPLY TO CRITICS. 27 



mainly water and the latter land, but because of the 

 enormous amount of heat transferred from the former 

 to the latter hemisphere by means of ocean-currents ; 

 and that, were it not for this transference, the tem- 

 perature of the water would exceed that of the land 

 hemisphere.* And it is in order to prove this that the 

 "four a priori reasons" which Professor Newcomb 

 criticises were adduced. The first of these is as 

 follows : — 



First. — 'The ground stores up heat only by the 

 slow process of conduction, whereas water, by the 

 mobility of its particles and its transparency for 

 heat -rays, especially those from the sun, becomes 

 heated to a considerable depth rapidly. The quantity 

 of heat stored up in the ground is thus comparatively 

 small, while the quantity stored up in the ocean is 

 great.' f 



These sentences are considered unworthy of criti- 

 cism. Are they really so unworthy ? Let us examine 

 them a little more closely. It is in consequence of 

 the sun's rays being able to penetrate to a great depth 

 that the amount of heat stored up by the ocean is so 

 great ; and it is to this store that its warmth during 

 winter is mainly due. The water is diathermanous 

 for the rays of the sun, but it is not so, for reasons 

 well known, for the rays of water itself. The upper 

 layers of the ocean will allow a larger portion of the 

 radiation from the sun to pass freely downward, but 

 they will not allow radiation from the layers under- 

 neath to pass freely upwards. These upper layers, 

 like the glass of a greenhouse, act as a trap to the 



* Since ' Climate and Time ' was published it has been proved 

 from observation (see next chapter) that, notwithstanding this trans- 

 ference of heat, the water hemisphere is the warmer of the two. 



f ' Climate and Time,' p. 90. 



