SOURCES OF HEAT. 211 



the surface of the earth occasions a difference in their heating effect is, that 

 the more the rays are inclined, the more tlier are diffused, or, in other words, 

 the larger the space they cover. This may be rendered apparent by reference 

 to Fig. 197. 



pjg jg'j Let us suppose A B C D to represent 



a portion of the Pun's rays, and C D a 

 ^ portion of the earth's surface upon which 



the rays fall perpendicularly, and C E 



portions of the surface upon which they 



^ fall obliquely. The same number of 



■^ rays will strike upon the surfaces C D 



and C E, but the surface C E being 

 greater than C D, the rays will necessarily fall more densely upon the latter ; 

 and as the heating power must be in proportion to the density of the rays, it 

 is obvious that C D will be heated more than C E, in just the same propor- 

 tion as the surface C E is more extended. But if we would compare two 

 surfaces upon neither of which the sun's rays fall perpendicularly, k't us take 

 C E and C F. They fall on C E with more obhquity than on C F; but C B 

 is evidently greater than C F, and therefore the rays being diffused over a 

 larger surface are less dense, and therefore less effective in heating. 

 What is the 473, The greatest natural temperature ever 

 ^ueSe?atur'e authentically recorded was at Bagdad, in 1819, 

 ever observed? when thc thermometer (Fahrenheit's) rose to 

 120° in the shade. On the west coast of Africa the ther- 

 mometer has been observed as high as 108° F. in the 

 shade. Burckhardt in Egypt, and Humboldt in South 

 Americaj observed it at 117° F. in the shade. 



474. About 70° below the zero of Fahrenheit's 

 lowest tempe- thermomctcr is the lowest atmospheric temper- 



rature observed? , . ii.ia,* • 



ature ever experienced by the Arctic navigators. 

 Towhatextent 475. Tho grcatcst artificial cold ever pro- 

 ^cZ beefprol ^uced was 220° F. below zero. 



*"c«<i ? This temperature was obtained some years since by M. 



Natterer, a German chemist. Professor Faraday also produced a cold 

 equal to 166° F. below zero. At neither of these temperatures were pure 

 alcohol or ether frozen. 



The temperature of the space above the earth's atmosphere has been esti- 

 mated at 58° below zero, Fahrenheit's thermometer. 



To what depth 476. The depth to which the influence of 

 does^^he^'hela ^^^ ^^^^^ ^^ ^^'^ ^^^^ cxtcnus iuto the earth va- 

 tend? ^""^ '^' "*^^ ^^^^ ^^ ^^ ^^^ ^^^^ > ii'^ver, however, ex- 

 ceeding the latter distance. 



