wa 
Variation of the Solar Radiation. 81 
the Mount Whitney Expedition :—“ Jf the absorption were 
invariable ut all hours of the day and for all parts of the earth 
situated at the same height above the sea and subjected to the 
same air-pressure, the comparison of high and low sun 
observations would give us the true energy outside the air, 
whether the absorbent material were distributed uniformly 
throughout the atmosphere or were gathered into horizonta) 
layers superposed according to any law whatever. The 
character of the atmosphere interposed between us and the 
sun, however, is constantly varying through the day ; and 
even if it were at rest, a vertical section would have a 
different composition from that made at a very great inclina- 
tion, which would necessarily pass over portions of the earth’s 
surface subjected to conditions very different from those 
existing at the place of observation.” 
These observations apply to the methods of reduction now 
employed, but owing to the increased accuracy of observa- 
tion, due to the great improvements of apparatus and 
especially to the writer’s introduction of the automatic or 
so-called bolographic recording devices, it is no longer 
necessary to follow the sun very long or to very great 
zenith distances, to obtain differences of ordinates ot the 
bolographs measurable with sufficient exactness. The deter-_ 
minations are now chiefly made between the hours of | and 
4 p.M., and it is only the air which transmits the rays during 
this interval, whose uniform transparency is in question. 
While the atmosphere extends to an indefinite elevation, the 
portions within a few miles of the earth’s surface contain 
almost all the variable constituents of it, and the path of 
the ray in these lower layers does not sweep over a very 
large extent. of the earth’s surface between the hours of 1 
and 4 P.M. 
The following table, computed for the times of the equi- 
noxes, gives the areas of the earth’s surface over which 
the lower air-layers must remain unaltered in transparency 
during the observations, if good results are to be obtained. 
Area over which constant In the air-iayer below an 
transparency is required. elevation of 
0:03 sq. miles. 1/5 mile. 
O86, ae arr 
3°0 i 2 miles 
6:8 “3 3 ” 
12:1 rs + 53 
3040 is | 20 3 
Phil. Mag. 8. 6. Vol. 8. No. 43. July 1904. e 
