THE RADIATION OF THE SUN—ABBOT. 145 
were solar radiation which was being measured. In such test ex- 
periments it is found that as much as 99 per cent of the heat in- 
troduced is recovered, and it is believed that the standard pyrhe- 
liometer gives the true scale of radiation for the sun within a prob- 
able error of a half of 1 per cent. The silver disk pyrheliometers 
have been compared with this standard, and in this way the standard 
scale of radiation has been diffused by the Smithsonian Institution, 
which has sent out about 25 copies of the standardized silver disk 
pyrheliometer to various countries 
of the world, in Europe and North 
and South America. 
Measurements with the pyrheli- 
ometer indicate that the maximum 
intensity of the sun’s radiation at 
sea level is about 1.5 calories per 
square centimeter per minute. At 
high-level stations, such as Mount 
Whitney, in southern California, at 
an altitude of 14,500 feet, the read- 
ings run as high as 1.7 calories per 
square centimeter per minute. You 
may ask why it is that if the inten- 
sity of the sun’s radiation increases 
as we go up a mountain, it should 
be also the case that the tempera- 
ture of the air at high elevations is 
- lower than it is at sea level. This 
is due to the property of the air of 
almost freely transmitting solar 
radiation. Like a pane of glass in = 
a window, it is not much warmed 
by absorbing the rays, whereas a 
blackened substance held in the 
beam of light, either upon a mountain or inside the window pane, 
will be very appreciably warmed. 
If we could go outside the atmosphere altogether, all the radia- 
tion which we receive from the whole sky, and which is derived 
by scattering, would be still in the direct sun-beam. Looking 
away from the sun we should see the stars shining, as if at night, 
and the sun’s rays themselves as observed by the pyrheliometer 
would exceed in intensity even those observed on high mountain sum- 
mits. Now the question is, what would be the intensity of the 
solar radiation if we could observe it outside the atmosphere, at 
the earth’s mean solar distance? This quantity is called the solar 
73176°—sM 1914——10 
a 
A) i 
Fig. 2.—Silver disk pyrheliometer. 
