434 Mr. J. Croll on the Change in the Obliquity of the Ecliptic, 



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 by 3,121,870 square miles of surface at the equa- 

 tor. The amount thus conveyed is equal to all the heat which 

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

 calculations made by Mr. Meech, the annual quantity of heat 

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

 of the whole zone, is ^~- of that received at the equator. Con- 

 sequently 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 consequence of the obliqueness of the sun's rays, 

 the total quantity of heat conveyed by the Gulf-stream will pro- 

 bably nearly equal the amount received from the sun by the 

 entire arctic regions. 



It may be stated, however, that the extra amount of heat ab- 

 sorbed in polar regions in consequence of the oblique direction 

 in which the rays pass through the atmosphere is not nearly so 

 great as is generally represented. It is commonly assumed 

 that the quantity of heat absorbed by the atmosphere is propor- 

 tionate to the number of aerial particles that the rays have to 

 encounter before reaching the surface of the earth. It is stated 

 as a general rule that, if the tracts of the rays follow an arith- 

 metical progression, the diminished force with which the rays 

 reach the ground will form a decreasing geometrical progression. 

 But recent discoveries in regard to the absorption of radiant 

 heat by gases and vapours prove that Tables computed on this 

 principle must be wholly erroneous. The researches of Tyndall 

 and Melloni show that, when a ray passes through any substance, 

 the absorption is rapid at first, but the ray is soon " sifted," as 

 it is called, and it then passes onwards with but little further ob- 

 struction. 



In the article on climate in the Encyclopedia Britannica, 



