172 
hypothesis will be transformed to heat in a single revolu- 
tion will be practically as before, i,e., 6'28 times the planet’s 
visible energy of motion at its mean distance from the sun. 
There will, however, be this difference between the two 
cases. In the instance of a body moving in a circular orbit 
the accession of heat will be a constant quantity in equal 
increments of time and in all parts of the orbit, whereas in 
the case of a body moving in an ellipse the accession will 
be least in approaching the perihelion and greatest in 
receding from it, inasmuch as the solar gravitation produces 
an increase of mechanical translation in the former instance 
and a diminution in the latter. Assuming the theory to be 
correct this circumstance is one which will greatly mitigate 
the extremes of heat to which, apart from such a considera- 
tion, we should expect cometic bodies to be subject. Indeed, 
in spite of the enormous distances to which they recede 
from the sun, the voluminous nature of their atmospheres 
by which radiation must be diminished, may, in conjunction 
with this view of the case, afford an explanation of the 
unexpectedly high temperature which, in the case of these 
bodies, the spectroscope certainly seems to indicate. 
The application of the views herein advanced to an 
explanation of the cause of solar lieat is briefly this 
As the mutual and ever-acting gravitation of the sun and 
his attendant planets produces in the instance of the central 
luminary no more acceleration of visible motion than in 
the instance of the planets themselves, the gravitation must 
result in the exhibition of some other form of energy, which in 
the one case as in the other will, I apprehend, be the form 
of heat. Carrying, however, this idea to the extreme, we 
should be led to infer that the force of gravitation, which in 
some circumstances is certainly capable of transference into 
the form of mechanical translation, and thence into the 
form of heat, is in others just as capable of transformation 
in the opposite order. Indeed, if heat is to be regarded as 
