160 Prof. P. Lowell on a Method for Evaluating 



the air, dust, water-globules, and ice, augment more rapidly 

 than the air-mass as one approaches the ground. 



Drawing the curve for transmission at the sea-level on the 

 same principles as those for outside the atmosphere at Gamp 

 Whitney and at Lone Pine, and then measuring the amounts 

 of transmission of each within the limits of the visual rays, 

 from A = 0*393//,, the K line, to A = 0*76yu,, the A band, we 

 get the following table : — 



Transmission of Solar Radiation in the Visible Spectrum. 



Calories received from Visible portion 



the whole spectrum. transmitted. 



' Outside the atmosphere 3*127 1*000 



Camp Whitney 1*896 0*664 



Lone Pine 1*533 0*482 



Sea-level 1*180 0*210 



The relative loss in the regions \ = 0*393//, to A = 0*76/* 

 and \ = 0'7 6 fjb to X = l°2 /uu between the several stations is as 

 follows : — 



I. II. 



Outside to Camp Whitney 0*105 0*029 



Camp Whitney to Lone Pine . . . 0*055 0*010 



Lone Pine to sea-level 0*086 0*027 



Light received from the Day Sky. — To these transmissions 

 must be added that part of the solar radiation which, scattered 

 by reflexion and diffraction in the atmosphere, is reflected 

 upon the ground causing the brightness of the day sky : it is 

 sufficient to obliterate the stars. Compared with direct sun- 

 light, its amount as determined hj Langley (' Professional 

 Papers of the Signal Service/ vol. xv.) is : 



Sun. Sky. 



Illumination 80 19 



or 24 per cent, of the Sun's light. We must therefore 

 increase the energy transmitted by 24 per cent, of itself. 

 This gives finally : — 



Same as affected Portion reflected 

 Transmission. by the daylight. into space. 



Outside 1*00 1*00 ' 



Sea-level ...... 0*21 0*26 0*74 



Albedo of the Earth.— Now the reflected fraction of the in- 

 cident energy in the visible spectrum is that by which we see 

 the body, and is called its albedo. The albedo of our air, then, 

 comes out 74. To get the whole albedo of the Earth we 

 must add to it the albedo of the surface. 



