and its Influence on Climate. 433 



been duly estimated. This will be seen if we merely consider 

 for a moment the effect produced by one current alone, viz. the 

 Gulf-stream. The total quantity of water conveyed by this stream 

 is probably equal to that of a stream 50 miles broad and 1000 

 feet deep, flowing at the rate of 4 miles an hour. And the mean 

 temperature of the entire mass of moving water is not under 

 65° at the moment of leaving the Gulf*. I think we are war- 

 ranted to conclude that the Gulf-stream, before it returns from 

 its northern journey, is on an average cooled down at least 25°. 

 Each cubic foot of water, therefore, in this case carries from the 

 tropics upwards of 1500 units of heat, or 1,158,000 foot-pounds. 

 According to the above estimate of the size and velocity of the 

 stream, 5,575,680,000,000 cubic feet of water are conveyed 

 from the Gulf per hour, or 133,816,320,000,000 cubic feet 

 dailyf. Consequently the total quantity of heat transferred 

 from the equatorial regions per day by the stream amounts to 

 154,959,300,000,000,000,000 foot-pounds. From observations 

 made by Sir John Herschel and by M. Pouillet on the direct 

 heat of the sun, it is found that, were no heat absorbed by the 

 atmosphere, about 83 foot-pounds per second would fall upon a 

 square foot of surface placed at right angles to the sun's rays J. 

 Mr. Meech estimates that the quantity of heat cut off by the atmo- 

 sphere is equal to about 22 per cent, of the total amount re- 

 ceived from the sun. M. Pouillet estimates the loss at 24 per 

 cent. Taking the former estimate, 64*74 foot-pounds per se- 

 cond will therefore be the quantity of heat falling on a square 

 foot of the earth's surface when the sun is in the zenith. And 

 were the sun to remain stationary in the zenith for twelve hours, 

 2,796,768 foot-pounds would fall upon the surface. 



It can be shown that the total amount of heat received upon 

 a unit-surface on the equator during the twelve hours from sun- 

 rise till sunset at the time of the equinoxes is to the total amount 

 which would be received upon that surface, were the sun to re- 

 main in the zenith during those twelve hours, as the diameter of 

 a circle to half its circumference, or as 1 to 1*5708. It follows, 

 therefore, that a square foot of surface on the equator receives 



* Phil. Mag. for February 1867, p. 127- 



t Captain Maury considers the Gulf-stream equal to a stream 32 miles 

 broad and 1200 feet deep, flowing at the rate of five knots (30,415 feet) 

 an hour (Physical Geography of the Sea,§ 24). This gives 6,166,700,000,000 

 cubic feet per hour as the quantity of water conveyed by the stream. Sir 

 John Herschel's estimate is still greater. He considers it equal to a stream 

 30 miles broad and 2200 feet deep, flowing at the rate of four miles an hour 

 (Physical Geography, § 54). This makes the quantity 7,359,900,000,000 

 cubic feet per hour. Sir John estimates the temperature at 86° F. 



% Trans, of Royal Soc. of Edin. vol. xxi. p. 57. Phil. Mag. S. 4. vol. 

 ix. p. 36. 



Phil Mag. S. 4. Vol. 33. No. 225. June 1867. 2 F 



