i88 5 .J 
123 
The Heat of the Sun. 
diameter is about no times greater. The bulk or cubic 
capacity of the Sun is therefore the cube of no, or 1,331,000 
times that of the Earth. But the mass, calculated from 
the attraction which it exerts, is only 360,000 times greater, 
— consequently the materials of -which the Sun is composed 
must be much lighter than those which compose our Earth. 
Speaking accurately and scientifically we should say that if 
the density of the Earth be taken as unity, that of the Sun 
is only 0*250 ; or if, as experiment shows, the mean specific 
gravity of ihe Earth is equal to 5*2, then that of the Sun is 
equal to 1*3. 
Having now fixed some of the physical constants of the 
centre of our planetary system, we have to determine the 
amount of heat which is given out. In the first place, it is 
evident that we who live upon the Earth must necessarily 
make our observations upon the Earth ; it is thus important 
that we should know what fraction of the total amount of 
heat emitted by the Sun is absorbed by the Earth. The 
radius of the Earth’s orbit, we have already seen, may be 
taken as 92,000,000 miles ; the circumference of the ap- 
proximate circle which is described around the Sun is thus 
equal to 3*1416 x 92,000,000 x 2, or about 578,000,000 miles. 
The diameter of the Earth is about 8000 miles ; conse- 
quently the Earth occupies about the 73,000th part of the 
circumference of the approximate circle which it describes 
around the Sun. But this refers only to one plane. Taking 
the area of the hollow shell surrounding the Sun at a dis- 
tance of 92,000,000 miles as equal to 115,000,000,000,000,000 
square miles, and taking the Earth as aflat plane 8000 miles 
in diameter, 01*50,000,000 square miles in area, we find that 
the Earth intercepts the 2,300,000,000th part of the total 
heat emitted by the Sun. If, then, we find the quantity of 
heat received by the Earth, and multiply it by this last 
number, we shall obtain the aCtual heat given out by 
the Sun. 
By allowing the rays from the Sun to fall perpendicularly 
upon a surface which is capable of absorbing them, and 
warming a known quantity of mercury, the heating power 
of the Sun has been determined. Two series of observa- 
tions on this point have been made : one at Paris by M. 
Pouillet, with his pyrheliometer ; and the other at the Cape 
of Good Hope by Sir John Herschell, with his adtinometer. 
The results are concordant, and they give that the amount 
of heat received by the Earth in one year would be capable 
of liquefying a layer of ice entirely surrounding the Earth 
to a depth of 100 feet. This, then, is the amount of heat 
