254 



HEAT. 



bear to II., and therefore III. is the curve he would have obtained had 

 he been outside the atmosphere altogether. The general absorption 

 steadily increases as the wave-length diminishes, though there is marked 

 absorption of particular rays in the infra-red. This can easily be seen by 

 inspection of the ratios of the ordinates of I. and II. at H with those at A. 



Curve III. , therefore, 

 contains a larger proportion 

 of the blue rays than I. or 

 II., and Langley concludes 

 that outside the atmosphere 

 the sun would appear of a 

 bluish tinge. Continuing 

 the curves beyond 1-0 as 

 far as he was able, and com- 

 paring the areas of curves 



1200 



llOO 



'000 



900 



\ 



800 



700 



600 



500 



400 



300 



200 



100 



I. and II., he found 

 Area high sun _ .. r _ 

 Area low sun 



Hence also area of curve 

 III. = 1'57 area of curve I. 

 Now by using Violle's actino- 

 meter, he obtained as the 

 total radiation received per 

 square centimetre per 

 minute at high sun on 

 clear days, 1'81 calories. 



Then outside the atmo- 

 sphere this would be in- 

 creased by the fraction 1-57, 

 giving 2 '84 calories. 



Langley believed that 

 even 2 '84 was an under- 

 estimate and put the solar 

 constant as at least 3. 



Crova's Researches. 

 Crova,* who devised and 

 carried on a series of re- 

 searches with an actino- 

 meter> put the value of the 

 constant still higher, be- 

 lieving that it is probably 

 as high as 4 calories per minute per square centimetre. 



Recent work tends to show that these values are too large. Abbott 

 and Fowle,f using the Bolometer, found almost the same value at Wash- 

 ington and at Mount Wilson, 1800 metres higher, just below 2'1 calories 

 per minute per square centimetre. Millochau,J working with the Fdry 

 pyrometer on the summit of Mont Blanc and at different levels below, 



* Cong. Int. de Phys., hi. p. 453, contains a discussion of the value of the solar 

 constant by M. Crova. 



f Terrestrial Magnetism, xiii. p. 79, 1908 ; Ast. Phys. Journ,, xxix. p. 281, 1909. 

 j Journ. de Physique, viii. p. 347, 1909. 



.3 



.40 



FIG. 146. Langley's Curves of Solar Radiation. 



I. High sun, through one atmosphere. 

 II. Low sun, i.e. through two atmospheres. 

 III. Estimated curve outside atmosphere. 



