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tempemture of the planet so lai* as its eonditioiis of I’adia- 
tion will permit. The aggregate value of the impulses 
required to make a body describe a circle and return to its 
primal position and direction without loss or gain of visible 
energy I have endeavoured to determine graphically by 
means of the mechanical parallelogram. The result at which 
I arrive is that the aggregate in question exceeds the mo- 
mentum of the body to be deviated in the same proportion 
as the circumference of a circle exceeds its radius, and this 
irrespective of the circle’s size. I have not attempted to 
calculate the temperature to which the matter of the earth 
would be elevated by a sudden stoppage in its orbital 
path, for it appears to me that satisfactory data for doing 
this do not exist. The number of thermal units to which 
such a stoppage would give rise if multiplied by 6 ’28 
indicate the amount of heat which upon the present 
hypothesis the earth annually receives through the in- 
strumentality of solar gravitation. This amount will^ 
I apprehend, am})ly account for a certain initial tempera- 
ture of the terrestrial matter, and for that excess of 
internal heat which appears to have been pretty well 
made out. It will be evident that any heat which in 
virtue of this hypothesis may be supposed to fall to the lot 
of our earth will in some calculable proportion fall to the 
lot of the other planets also. That proportion I apprehend 
to be directly as the planet’s mass multiplied into its velocity 
in orbit and divided by its periodic time. Of course this 
amount of heat will be distributed over the entire matter of 
the planet, the temperature of which will depend on the 
capacity for heat of the matter composing it. and on its 
facilities for radiation. These latter will clearly diminish 
with the comparative diminution of superficies enjoyed 
by the larger planets and with the extension of their 
atmospheres. In the case of bodies moving in elliptic 
orbits the amount of energy which according to this 
