Variation of the Solar Radiation. 57 
II. 
Possibility of Considerable Changes in the Amount of Solar 
Radiation outside the Atmosphere. 
Looking at the general results, these seem then to indicate 
a possibility that a Ho fall of solar radiation occurred 
about the close of March*, and that subsequently the radia- 
tion continued nearly or quite 10 per cent. less than before. 
This, if certain, would be important, and we may inquire 
what causes on the sun could produce such a change, and 
what effects might be expected to be produced on the earth 
if it occurred. 
The writer showed nearly thirty years ago+ that the en- 
velope of the sun profoundly influences by its absorption the 
radiation received by the earth. While the absorption in the 
solar envelope is not exactly known, still so much is known 
that we may infer that if it were absent for a moment, the 
earth would receive nearly double its present amount of heat, 
If a variation of 10 per cent. in the transparency of this 
envelope occurred, nearly 10 per cent. of change in the solar 
radiation outside the earth’s atmosphere would follow. 
Tf a change of solar radiation did occur, there ought to 
have been a similar fall of terrestrial temperatures afterwards; 
and we may inquire how great this fall of temperature 
should be. 
The earth may be regarded as a body at a mean tempera- 
ture of 290° absolute (17° C.) maintained at approximately 
constant temperature by a balance between solar radiation 
received and terrestrial radiation emitted. It is here as- 
sumed that all sources of heat other than the solar radiation 
are negligible, but if any or all of them are not so, their 
presence will be to reduce the effect on temperature, of a fall 
in solar radiation. 
Recent studies of German physicists have experimentally 
verified, for the perfect radiator, Stefan’s law that the emission 
of a heated body is proportional to the fourth power of the 
temperaturet. Other bodies not perfect radiators depart 
from this law in the sense that while radiating less abso- 
lutely than the perfect radiator, their emission is more nearly 
proportional to a power of the’ temperature higher than the 
* It is of interest to note that a marked increase of sunspots occurred 
on March 21. See Report of the Council, Royal Astronomical Society 
Monthly Notices, vol. lxiv. p. 357. 
+ Comptes Rendus, Sept. 6, 1875. 
t O. Lummer, Rapports présentés au Congres International de Physique, 
1900, vol. ii. pp. 78-81. . 
