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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;f 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 energy \ 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 Rankine 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 



* Herschel's Astronomy, Edition 1833. — See last Ed., § (399). 



f Joule " On the Generation of Heat in the Galvanic Circuit," 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 1843, and published, Phil. Mag., Sept. 1843. " On 

 the Changes of Temperature produced by the R-arefaction and Condensation of Air," commu- 

 nicated to the Royal Society, June 1844, and published, Phil. Mag., May 1845. Joule and Scoresby 

 " 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 Rankine has called it. 

 VOL. XXI. PART I. S 



