On the other hand, the dynamical energy of luminiferous vibrations was referred to as the mechanical power allotted to

rooms or funguses, which can

plants (not mush

grow in the dark,

are nourished by organic food like animals, and like animals absorb oxygen and exhale carbonic acid), to enable them to draw carbon from carbonic acid, and hydrogen from water.

In conclusion, the sources available to man for the production of mechanical effect were examined and traced to the sun's heat and the rotation of the earth round its axis.

Published speculations 1 were referred to, by which it is shown to be possible that the motions. of the earth and of the heavenly bodies, and the heat of the sun, may all be due to gravitation; or that the potential energy of gravitation may be in reality the ultimate created antecedent of all motion, heat, and light at present existing in the universe.1

1 Trans. Roy. Soc. Edinburgh, April, 1854 Professor W. Thomson, "On the Mechanical Energies of the Solar System." Also British Association Report, Liverpool, September, 1854 "On the Mechanical Antecedents of Motion, Heat, and Light." (Republished in Vol. II. of Mathematical and Physical Papers).

ON THE SOURCES OF ENERGY IN

NATURE AVAILABLE TO MAN
FOR THE PRODUCTION OF

MECHANICAL EFFECT.

[From Presidential Address to the Mathematical and Physical Section of the British Association, York, 1881.]

DURING the fifty years' life of the British Association, the Advancement of Science for which it has lived and worked so well, has not been more marked in any department than in one which belongs very decidedly to the Mathematical and Physical Section-the science of Energy. The very name energy, though first used in its present sense by Dr. Thomas Young about the beginning of this century, has only come into use practically after the doctrine which defines it had, during the first half of the British Association's life, been raised from a mere formula of mathematical

VOL. II.

F F

dynamics to the position it now holds of a principle pervading all nature and guiding the investigator in every field of science.

A little article communicated to the Royal Society of Edinburgh a short time before the commencement of the epoch of energy under the title "On the Sources Available to Man for the Production of Mechanical Effect "1 contained the following:--

"Men can obtain mechanical effect for their "own purposes by working mechanically themselves, and directing other animals to work for them, or by using natural heat, the gravitation "of descending solid masses, the natural motions. "of water and air, and the heat, or galvanic "currents, or other mechanical effects produced "by chemical combination, but in no other way "at present known. Hence the stores from I which mechanical effect may be drawn by "man belong to one or other of the following classes :

66

1 Read at the Royal Society of Edinburgh on February 2, 1852: Published in Proceedings of that date; and republished in Vol. I. of Mathematical and Physical Papers.

"I. The food of animals.

"II. Natural heat.

"III. Solid matter found in elevated posi❝tions.

“IV. The natural motions of water and air.

"V. Natural combustibles (as wood, coal coal-gas, oils, marsh-gas diamond, native sul"phur, native metals, meteoric iron).

"VI. Artificial combustibles (as smelted or electrically-deposited metals, hydrogen, phos

"phorus).

"In the present communication, known facts "in natural history and physical science, with "reference to the sources from which these stores "have derived their mechanical energies, are "adduced to establish the following general conclusions :

"I. Heat radiated from the sun (sunlight being "included in this term) is the principle source of "mechanical effect available to man.1 From it is "derived the whole mechanical effect obtained by

1 A general conclusion equivalent to this was published by Sir John Herschel in 1833. See his Astronomy, edit. 1849, § (399).

"means of animals working, water-wheels worked “by rivers, steam-engines, galvanic engines, windmills, and the sails of ships.

2. The motions of the earth, moon, and sun, and their mutual attractions, constitute an im"portant source of available mechanical effect. "From them all, but chiefly no doubt from the "earth's motion of rotation, is derived the me"chanical effect of water-wheels driven by the "tides.

3. The other known sources of mechanical effect available to man are cither terrestrial — "that is, belonging to the earth, and available "without the influence of any external body—or "meteoric—that is, belonging to bodies deposited "on the earth from external space. Terrestrial “sources, including mountain quarries and mines, "the heat of hot springs, and the combustion “of native sulphur, perhaps also the combustion "of inorganic native combustibles, are actually used; but the mechanical effect obtained from "them is very inconsiderable, compared with "that which is obtained from sources belonging