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motion thus communicated to the body in question is not 
equal to the sum of the impulses, being, as is well known, 
justly represented by the diagonal of a parallelogTam the 
length of whose sides is proportioned to the value of tlie 
primal impulses. What then in this case becomes of the 
residue of energy applied and not converted into visible 
motion ? I apprehend it assumes the form of heat. If so 
in the instance of a body receiving equal impulses in direc- 
tions inclined to each other at an angle of 120 degrees, the 
resulting visible motion will just account for the energy of 
one of these impulses, and the resulting heat for the energy 
of the other. If then a body having a certain amount of 
visible energy observable as rectilinear movement have a 
series of impulses imparted at an angle of 120 degrees in 
each instance to the line of motion and equal in value to 
the visible energy of the body at the time of application, 
the body in question will gain nothing in visible motion 
through the application of these impulses, but will simply 
alter the direction of its movement at each application and 
acquire at the same time an amount of heat of which the im- 
pulse is the mechanical equivalent. Adopting this view, it 
matters not at what angle the impulse may be given, for so 
long as the figure representing its amount yields with that 
representing the visible motion of our body a parallelogram 
whose resultant is equal to the line of visible motion, it fol- 
lows that the energy of mechanical translation remains 
unaltered and the energy of the impulse becomes wholly 
transformed into heat. This condition of things however is 
what obtains in the instance of a body in planetary revolu- 
tion in a circular orbit. It is continually receiving impulses 
through the instrumentality of gravitation at or nearly at 
right angles to its line of flight, and its energy of visible 
motion does not increase, whence I infer that the mechani- 
cal energy by which its rectilinear movement is destroyed 
is converted into an equivalent of heat which elevates the 
