Otago Institute, 487 
Mr. Waterston used one of these instruments in India, and determined 
the singular fact that, whatever the temperature of the environment, the 
difference between it and that of the thermometer exposed to direct solar 
radiation was always the same. His experiments were made with tempera- 
tures varying from 60° Fah. to 220° Fah. 
Father Secchi, for the basis of his calculations, adopted the experiments 
made by M. Soret on Mont Blane and in other mountainous regions, in 
order that he might avoid as much as possible the errors introduced into the 
results by atmospheric influences, He obtained a difference of 29-02 Cent. 
(= 52°24 Fah.) between the temperature of the thermometer and that of 
the enceinte. 
M. Pouillet appears to have obtained a somewhat higher figure. Mr. 
Ericsson, whose observations have decidedly been conducted with greater 
care to avoid disturbing influences, and with more completeness than those 
of any other observer, arrived at 67:20° Fah., as the effective intensity of 
solar radiation at aphelion, and 72-68° Fah. for perihelion. The difference 
between these two temperatures coincides very closely with that which he 
has calculated as the necessary effect of the nearer approach of the earth 
to the sun at the latter period of the year. 
The figures thus ascertained require correction (1) for the absorption by 
the earth’s atmosphere, which is approximately known ; and (2) for that of 
the sun’s absorption, as to which the widest differences of opinion exist. It 
then remains to determine what the temperature of a body must be which can 
radiate so large a quantity of heat across the space which divides the sun 
from the earth. Here, again, irreconcilable differences of opinion exist 
as to the law of radiation. Pouillet, and with him a number of eminent 
French physicists, have adopted the law of cooling established by Dulong 
and Petit. According to this, the radiation increases so much more rapidly 
than the temperature, that an increase of 600° in temperature multiplies a 
hundredfold the energy of radiation. Using this law Pouillet fixed the 
temperature of the sun’s surface, or rather that portion of it which is 
effectively radiated into space, at from 1,461° to 1,761° Cent, i= 2,680 to 
3,170 Fah). Vicaive, adopting Secchi’s value of the solar radiation, obtains 
by the same law a temperature of 1,898° Cent., or about 2,520° Fah., and 
estimates that, when all the necessary corrections have been made, the 
result must still be less than 8,000° Cent.—say 5,400° Fah. 
Both Secchi and Ericsson refuse to accept Dulong’s law, and fall back 
on that of Newton, who assumed that the intensity of radiation from a 
hotter body to a cooler one must be proportionate to the differences of their 
temperatures and to the distance between them. The latter element of the 
calculation is of course treated in the same manner by both parties. Pro- 
