438 Proceedings. 
ceeding in this matter, Ericsson fixes the solar temperature at 4,085,584° 
Fah.; whilst Father Secchi makes it at least 18,000,000° of the same 
scale. The widest difference in their treatment of the question lies in ther 
respective estimates of the absorbing power of the sun’s atmosphere. af 
need not stop to consider the arguments they adduce, each in support ot his 
own view. So far as they differ on other points, I have no hesitation n 
accepting Ericsson’s results as the more reliable of the two. As to the in- 
fluence of the absorbing media in the neighbourhood of the sun, there can 
be no doubt that it is much better, in the present state of our knowledge, 
to limit our investigation to a search for the value of the effective xalltataey 
instead of seeking to calculate what actual internal temperature this must 
indicate. 
We find, then, that the basis of all these various calculations of the 
temperature of the sun is the ascertained difference between the tempera 
ture established in a terrestrial object on which the rays of the sun shine 
directly, and the general temperature of surrounding objects more or ee 
completely screened from those rays. It appears to me that the indications 
thus trusted to are not satisfactory definitions of the work which the solar 
rays are actually performing. They fall very far short of this, because 
what we want to know is not the difference which has thus been measured, 
but the difference between the actual temperature the sun’s rays can create 
in terrestrial objects, and the temperature at which those objects would rest 
if the heat radiated from the sun were withdrawn. : 
The earth itself is a heated body, and a certain temperature would exist 
at its surface if the solar radiation ceased entirely. This temperature would 
steadily fall, in consequence of the earth’s own radiation into space; - 
what we need to ascertain is the initial temperature for the moment of with- 
drawal of the solar heat, if such an event could happen. The difference of 
this temperature, and the highest which a thermometer will indicate when 
‘subjected to the action of a vertical sun on the clearest day, with a corrective 
introduced for the (approximately known) absorption of the terrestrial 
atmosphere, is the nearest measure we can obtain of the actual solar radia- 
tion. Evidently this quantity will greatly exceed any of those which have 
hitherto been adopted by physicists. 
It can hardly prove impossible to determine how much of the average? 
temperature at the surface of the earth is due to the solar heat and how 
much to the internal heat of the earth. The condition of affairs produced 
by our long polar winter nights offers us data in one direction. ‘That which 
‘obtains in arid equatorial wastes will serve us in the other direction. Eve 
when all the necessary data are collected, the problem will not be easy of 
solution ; but an approximate estimate cannot be beyond the power of our 
