526 PROFESSOK J. H. POYNTING ON RADIATION IN THE SOLAR SYSTEM: 
The, Constant of Radiation. 
If E is the energy radiated per second per square centimetre by a full radiator at 
temperature 6° A (where A stands for the absolute scale), the fourth-power law states 
that 
E = cre\ 
where o- is the constant of radiation. 
Accordino- to Kurlbaum"^' the constant is 
(7 = 5-32 X 10-= erg. 
The Solar Constant. 
The solar constant is usually expressed as a number of calories received per minute 
by a square centimetre held normal to the sun’s rays at the distance of the earth. 
The determinations by different observers differ so widely that it is not necessary for 
our present purpose to consider whether the constant really exists or whether there 
are small periodic variations from constancy. 
Angstrom estimated the value as 4 calories per square centimetre per minute, and 
this value is adopted by Croya as very probable.! When converted to ergs per 
second this gives 
= 0'28 X lO"^ ergs/cm." sec., 
o 
where the sufiix denotes that it is Angstrom’s value. 
Langley.|. assumed that the atmosphere transmits about 59 per cent, of the energy 
from a zenith sun and from his measurement of the heat reaching the earth’s surface 
he estimated the value of the constant at 3 calories. This gives 
S/ = 0'21 X 10''ergs/cm.® sec., 
Eosetti§ assumed a transmission of 78 per cent, from the zenith sun, but Wilson and 
GrayII consider that 71 per cent, represents Eosetti’s numhers better than 78 per cent. 
If in Langley’'’s value we replace 59 per cent, by 71 per cent, we get 2n calories. 
This gives 
S,. = 0'175 X 10’'ergs/cm.^ sec. 
‘ Wied. Ann.,’ vol. 65, 1898, p. 748. 
t ‘ Congres International de I’hysique,’ vol. 3, p. 453. 
t ‘ Phil. Mag.,’ vol. 15, 1883, p. 153, and ‘Researches on Solar Heat.’ 
§ ‘ Phil. Mag.,’ vol. 8, 1879, p. 547. 
II ‘Phil. Trans.,’ A, 1894, p. 383. 
