the Distribution of Heat over the Globe. 99 



low an arithmetical progression, the diminished force with which 

 the rays reach the ground will form a decreasing geometrical 

 progression. According to this Table about 75 per cent, of 

 the sun's rays are cut off by the atmosphere in arctic regions. 

 But if 75 per cent, of the rays were cut off by the atmosphere 

 in arctic regions, then the direct rays of the sun could not main- 

 tain a mean temperature 100° above that of space. But this is 

 no doubt by far too high a percentage for the quantity of heat 

 cut off; for recent discoveries in regard to the absorption of ra- 

 diant heat by gases and vapours prove that Tables computed on 

 this principle must be incorrect. The researches of Tyndall 

 and Melloni show that when rays pass through any substance, 

 the absorption is rapid at first ; but the rays are soon " sifted/'' 

 as it is called, and they then pass onwards with but little fur- 

 ther obstruction. Still, however, owing to the dense fogs which 

 prevail in arctic regions, the quantity of heat cut off must be 

 considerable. If as much as 50 per cent, of the sun's rays are 

 cut off by the atmosphere in arctic regions, the amount of heat 

 received directly from the sun is not sufficient to maintain a 

 mean annual temperature of — 100°. Consequently the arctic 

 regions must depend to an enormous extent upon ocean-cur- 

 rents for their temperature. 



Influence of Ocean-currents shown by a Second method. — That 

 the temperature of the arctic regions would sink enormously and 

 the temperature of the equator rise enormously were all ocean- 

 currents stopped, can be shown by another method, viz. by taking 

 the mean annual temperature from the equator to the pole along 

 a meridian passing through the ocean, say, the Atlantic, and. 

 comparing it with the mean annual temperature taken along a 

 meridian passing through a great continent, say, the Asiatic. 



Professor J. D. Forbes, in an interesting memoir*, has endea- 

 voured by this method to determine what would be the tempe- 

 rature of the equator and the poles were the globe all water or 

 all land. He has taken the temperature of the two meridians 

 from the Tables and charts of M. Dove, and ascertained the 

 exact proportion of land and water on every 10° of latitude from 

 the equator to the poles, with the view of determining what pro- 

 portion of the average temperature of the globe in each parallel 

 is due to the land, and what to the water which respectively be- 

 longs to it. He next endeavours to obtain a formula for ex- 

 pressing the mean temperature of a given parallel, and thence 

 arrives at "an approximate answer to the inquiry as to what 

 would have been the equatorial or polar temperature of the globe, 

 or that of any latitude, had its surface been entirely composed of 

 land or water/' 



* Trans, of Roy. Soc. Edinb. vol. xxii. p. 75. 

 H2 



