128 Mr. J. Croll on the Excentricity of the Earth's Orbit, 



four miles an hour from every part of the equator over the 

 northern hemisphere towards the pole. If its velocity were equal 

 to that of a good sailing-breeze, which Sir John Herschel states 

 to be about twenty-one miles an hour, the current would require 

 to be above 1200 feet deep. The Gulf-stream, before it returns 

 from its northern journey, is cooled down to about 40°. Each 

 cubic foot of water therefore carries from the tropics upwards 

 of 1500 units of heat. A greater quantity of heat is probably 

 conveyed by the Gulf-stream alone from the tropical to the tem- 

 perate and arctic regions than by all the aerial currents which 

 flow from the equator. 



During the warm periods of the glacial epoch (that is, when 

 the glaciation prevailed in the southern hemisphere), the quan- 

 tity of warm water flowing from the tropical to the arctic re- 

 gions would, as we have already seen, far exceed that which is 

 being transferred at present. The effect that this would have 

 along with the short and warm winter in melting the polar ice 

 would be enormous. At that period Greenland, in all probability, 

 would be free of ice. 



We are apt, on the other hand, to overestimate the amount 

 of heat conveyed from tropical regions to us by means of aerial 

 currents. The only currents which flow from the equatorial 

 regions are the upper currents or anti-trades, as they are called. 

 But it is not possible that much heat can be conveyed to us 

 directly by them. The upper currents of the trade-winds, even 

 at the equator, are nowhere below the snow-line. They must 

 therefore lie in a region actually below the freezing-point. In 

 fact, if those currents were warm, they would elevate the snow- 

 line above themselves. The heated air rising off the hot burn- 

 ing ground at the equator, after ascending for a few miles, 

 becomes exposed to the intense cold of the upper regions of the 

 atmosphere. It then very soon loses all its heat, and returns 

 from the equator much colder than it came. It is impossible 

 that we can receive any heat directly from the equatorial re- 

 gions by means of aerial currents. It is perfectly true that the 

 south-west wind, to which we owe so much of our warmth in 

 this country, is a continuation of the anti-trade. But the heat 

 which this wind brings to us is not derived from the equa- 

 torial regions. This will appear evident, if we but reflect that, 

 before the upper current descends to the snow-line after leaving 

 the equator, it must traverse a space of at least 2000 miles; 

 and to perform this long journey several days will be required. 

 During all this time the air is in a region below the freezing- 

 point ; and it is perfectly obvious that by the time it begins to 

 descend it must have acquired the temperature of the region in 

 which it has been travelling. 



