194 SPARKS FROM A GEOLOGIST'S HAMMER. 



at present exerts upon the climate of the northern hemis- 

 phere would, on the hypothesis of an extended northern 

 snow-cap, be completely withdrawn. 



To what does this influence amount? Mr. Croll has 

 shown that about one-fifth of all the heat possessed by 

 the waters of the North Atlantic, within the limits of the 

 North Temperate Zone, is derived from the Gulf Stream. 

 According to Dove, the mean temperature of London is 

 10 above the normal tem2:)erature of that parallel of lati- 

 tude. This excess has been justly attributed to the influence 

 of the Gulf Stream. But this by no means measures the 

 absolute influence of the Gulf Stream. This current, with 

 the other outgoing currents from the tropical zone, raises 

 the general temperature of the North Temperate Zone, 

 so that the normal temperature of the London parallel i.s 

 30 above the temperature which would he normal were 

 all the ocean currents arrested. The absolute influence 

 of the Gulf Stream upon the climate of London is repre- 

 sented, therefore, by 30+10 = 40. A depression of the 

 mean temperature of London to this extent would consti- 

 tute a serious modification of its climate. 



Now, in accordance with the theory here under con- 

 sideration, the reduction of London temperature which 

 must result from the arrest of the Gulf Stream would 

 take place precisely when the intensity of the solar radi- 

 ation would be diminished one-fifth, and the winter season 

 prolonged 36 daj^s. Let these three causes of a climatic 

 chill concur, and it becomes easy to admit that the wintry 

 precipitation of Great Britain and all northern Europe 

 must be in the form of snow, and in such amounts as to 

 outlast, like the living Alpine glaciers, the dissolving ac- 

 tion of the intensest summer sun. We seem, therefore, to 



