tlie Distributioa of Heat over the Globe. 83 



ferred from the equatorial regions per day by the stream amounts 

 to 154,959,300,000,000,000,000 foot-pounds. 



This estimate of the volume of the stream is considerably less 

 than that given both by Captain Maury and by Sir John Her- 

 schel. Captain Maury considers the Gulf-stream equal to a 

 stream 32 miles broad and 1200 feet deep, flowing at the rate of 

 five knots an hour (Physical Geography of the Sea, § 24, 6th 

 edition). This gives 6,165,700,000,000 cubic feet per hour as 

 the quantity of water conveyed by this stream. Sir John Her- 

 scheFs 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. 



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*. Mr. Meech estimates that the quan- 

 tity of heat cut off by the atmosphere is equal to about 22 per 

 cent, of the total amount received from the sun. M. Pouillet 

 estimates the loss at 24 per cent. Taking the former estimate, 

 64*74 foot-pounds per second 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 

 from the sun at the time of the equinoxes 1,780,474 foot-pounds 

 daily, and a square mile 49,636,750,000,000 foot-pounds daily. 

 But this amounts to only l * part of the quantity of heat 

 daily conveyed from the tropics by the Gulf-stream. In other 

 words, the Gulf-stream conveys as much heat as is received from 

 the sun b'y 3,121,870 square miles at the equator. The amount 

 thus conveyed is equal to all the heat which falls upon the globe 

 within 63 miles on each side of the equator. According to cal- 

 culations made by Mr. Meechf, the annual quantity of heat re- 

 ceived by a unit surface on the frigid zone, taking the mean 



* Trans, of Roy. Soc. of Edinb. vol. xxi. p. 57. Phil. Mag. S. 4. vol. ix. 

 p. 36. 



t Smithsonian Contributions to Knowledge, vol. ix. 



G2 



