COGS +3) 
VII.—On the Mechanical Energies of the Solar System. 
By Professor WILLIAM THOMSON. 
(Read 17th April 1854.) 
The mutual actions and motions of the heavenly bodies have long been 
regarded as the grandest phenomena of mechanical energy in nature. Their 
light has been seen, and their heat has been felt, without the slightest sus- 
picion that we had thus a direct perception of mechanical energy at all. Even 
after it has been shewn* that the almost inconceivably minute fraction of the Sun’s 
heat and light reaching the earth is the source of energy from which all the me- 
chanical actions of organic life, and nearly every motion of inorganic nature at 
its surface, are derived, the energy of this source has been scarcely thought of as 
a development of mechanical power. 
Little more than ten years ago the true relation of heat to force, in every 
electric, magnetic, and chemical action, as well as in the ordinary operations of 
mechanics, was pointed out;} and it is a simple corollary from this that the Sun, 
within the historical period of human observation, has emitted hundreds of times 
as much mechanical energyt as that of the motions of all the known planets taken 
together. The energy, that of light and radiant heat, thus emitted, is dissipated 
always more and more widely through endless space, and never has been, pro- 
bably never can be, restored to the Sun, without acts as much beyond the scope 
of human intelligence as a creation or annihilation of energy, or of matter itself, 
would be. Hence the question arises, What is the source of mechanical energy, 
drawn upon by the Sun, in emitting heat, to be dissipated through space? In 
speculating on the answer, we may consider whether the source in question con- 
sists of dynamical energy, that is, energy of motion, § or of “potential energy,” 
(as Mr Ranxrnz has called the energy of force acting between bodies, which will 
give way to it unless held); or whether it consists partly of dynamical and partly 
of potential energy. 
And again, we may consider whether the source in question, or any part of 
it, is in the Sun, or exists in surrounding matter, until taken and sent out again 
* Herscuer’s Astronomy, Edition 1833.—See last Ed., § (399). 
_ + Jovzz “On the Generation of Heat in the Galvanic Cireuit,” communicated to the Royal Society 
of London, Dec. 17, 1840, and published, Phil. Mag., Oct. 1841. “On the Heat evolved during 
the Electrolysis of Water,” Literary and Phil, Soc. of Manchester, 1843, Vol. vii., Part 3., Second 
Series. “On the Calorific Effects of Magneto-Electricity, and the Mechanical Value of Heat,” com- 
municated to the British Association, August 18438, and published, Phil. Mag., Sept. 1843. ‘“ On 
the Changes of Temperature produced by the Rarefaction and Condensation of Air,” commu- 
nicated to the Royal Society, June 1844, and published, Phil. Mag., May 1845. Jouve and Scorzssy 
“On the Powers of Electromagnetism, Steam, and Horses,’ Phil. Mag., June 1846. 
+ Once every 20 years or so.—See Table of Mechanical Energies of the Solar System, appended. 
§ “ Actual energy,” as Mr Ranxve has called it. 
VOL. XXI. PART I. S 
