160 /. Croll — Influence of the Gulf-stream. 



of heat received by a unit surface on the frigid zone, taking the mean 

 of the whole zone, is ^-^V of that received at the equator. Conse- 

 quently the quantity of heat conveyed by the Gulf-stream in one 

 year is equal to the heat which falls on an average on 6,873,800 

 square miles of the arctic regions. The frigid zone or arctic regions 

 contain 8,130,000 square miles. There is actually, therefore, nearly 

 as much heat transferred from the tropical regions by the Gulf-stream 

 as is received from the sun by the entire arctic regions ; the quantity 

 conveyed by the stream to that received from the sun by those 

 regions being as 15 to 18. 



But we have been assuming in our calculations that the per- 

 centage of heat absorbed by the atmosphere is no greater in polar 

 regions than it is at the equator, which is not the case. If we make 

 due allowance for the extra amount absorbed in polar regions in con- 

 sequence of the obliqueness of the sun's rays, the total quantity of 

 heat conveyed by the Gulf-stream will probably nearly equal the 

 amount received from the sun by the entire arctic regions. 



If we compare the quantity of heat conveyed by the Gulf-stream 

 with that conveyed by means of aerial currents, the result is equally 

 startling. The density of air to that of water is as 1 to 770, and its 

 specific heat to that of water is as 1 to 4-2. Consequently the same 

 amount of heat that would raise 1 cubic foot of water 1* would raise 

 770 cubic feet of air 4^-2, or 3234 cubic feet 1°. The quantity of 

 heat conveyed by the Gulf-stream is therefore equal to that which 

 would be conveyed by a current of air 3234 times the volume of the 

 Gulf-stream, and at the same temperature and moving with the 

 same velocity. Taking, as before, the width of the stream at 50 

 miles, and its depth at 1000 feet, and its velocity at 4 miles an hour, 

 it follows that in order to convey an equal amount of heat from the 

 tropics by means of an aerial current, it would be necessary to have 

 a current about 1^ mile deep and at the temperature of &5° blowing 

 at the rate of 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. A greater quantity of heat is probably con- 

 veyed by the Gulf-stream alone from the tropical to the temperate 

 and arctic regions than by all the aerial currents which flow from the 

 equator. 



We are apt, on the other hand, to over-estimate 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 no- 

 where 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 burning ground at the equator, after 

 ascending for a few miles, becomes exposed to the iatense cold of the 



