SCIENCE. 



475 



Laboratory at the Massachusetts Institute of Techno- 

 logy, and such-like enterprises are doing much in this 

 direction. To-day Chemistry and Physics are taught 

 in nearly all the academies and high schools of the 

 land j so that the larger colleges, whenever they see 

 fit, may easily require from the candidate for admis- 

 sion a wider knowledge of these sciences than they 

 themselves taught a dozen years ago. When and in 

 what manner the present scientific movement shall 

 culminate, no one can say ; but the fact of growth is 

 evident everywhere. This report is an attempt to 

 catch the present aspect of affairs and fix it in a per- 

 manent record. 



ON THE SOURCES OF ENERGY IN NATURE 

 AVAILABLE TO MAN FOR THE PRODUC- 

 TION OF MECHANICAL EFFECT* 

 By Sir William Thomson, F. R. S. 



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 depart- 

 ment 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 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 Avail- 

 able to Man for the Production of Mechanical Effect,"! 

 contained the following : 



" Men can obtain mechanical effect for their own pur- 

 poses 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 cur- 

 rents, or other mechanical effects produced by chemical 

 combination, but in no other way at present known. 

 Hence the stores from which mechanical effect may be 

 drawn by man belong to one or other of the following 

 classes : 



" I. The food of animals. 



" II. Natural heat. 



" III. Solid matter found in elevated positions. 



" IV. The natural motions of water and air. 



"V. Natural combustibles (as wood, coal, coal-gas, 

 oils, marsh-gas, diamond, native sulphur, native metals, 

 metoric iron.) 



"VI. Artificial combustibles (as smelted or electric- 

 ally-deposited metals, hydrogen, phosphorus). 



" In the present communication, known tacts 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 in- 

 cluded in this term) is the principal source of 7>techanical 

 effect available to man,\ From it is derived the whole 



* British Association, 1881. 



t Read at the Royal Society of Edinburgh on February 2, 1852. 

 {Proceedings of that date.) 



% A general conclusion equivalent to this was published by Sir John 

 Herschel in 1833. See his " Astronomy," edit. 1849, § (399.) 



mechanical effect obtained by means of animals work- 

 ing, water-wheels worked by rivers, steam-engines, gal- 

 vanic engines, wind-mills, and the sails of ships. 



" 2. The motions of the earth, moon, and sun, and 

 their mutual attractions, constitute an important source 

 of available mechanical effect. From them all, but 

 chiefly no doubt from the earth's motion of rotation, is 

 derived the mechanical effect of water-wheels driven by 

 the tides. 



" 3. The other known sources of mechanical effect 

 available to man are either 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. The terres- 

 trial 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 ef- 

 fect obiained from them is very inconsiderable, compared 

 with that which is obtained from sources belonging to 

 the two classes mentioned above. Meteoric sources, in- 

 cluding only the heat of newly-fallen meteoric bodies, 

 and the combustion of meteoric iron, need not be reck- 

 oned among those available to man for practical pur- 

 poses." 



Thus we may summarize the natural sources of en- 

 ergy as Tides, Food, Fuel, Wind and Rain. 



Among the practical sources of energy thus exhaust- 

 ively enumerated, there is only one not derived from sun- 

 heat — that is the tides. Consider it first. I have called 

 it practical, because tide mills exist, but the places where 

 they can work usefully are very rare, and the whole 

 amount of work actually done by them is a drop to the 

 ocean of work done by other motors. A tide of two 

 meters' rise and fall, if we imagine it utilized to the ut- 

 most by means of ideal water wheels doing, with perfect 

 economy, the whole work of filling and emptying a dock 

 basin in infinitely short times, at the moments of 

 high and low water, would give just one metre-ton per 

 square metre of area. This work done four times in the 

 twenty-four hours, amounts to 1.1620th of the work of 

 a horse-power. Parenthetically, in explanation, I may 

 say that the French metrical equivalent (to which in all 

 scientific and practical measurements we are irresistibly 

 drawn, notwithstanding a dense barrier of insular preju- 

 dice most detrimental to the islanders), — the French met- 

 rical equivalent of James Watt's " horse-power " of 550 

 foot-pounds per second, or 33,000 foot-pounds per min- 

 ute, or nearly 2,000,000 foot-pounds per hour, is 75 

 metre-kilogrammes per second, or i, l / 2 rretre-tons per 

 minute, or 270 metre-tons per hour. The French ton 

 of 1000 kilos, used in this reckoning, is 0.984 of the Brit- 

 ish ton. 



Returning to the question of utilizing tidal energy, we 

 find a dock area of 162,000 square metres (which is little 

 more than 400 metres square) required for 100-horse 

 power. This, considering the vast costliness of dock 

 construction, is obviously prohibitory of every scheme for 

 economizing tidal energy by means of artificial dock 

 basins, however near to the ideal perfection might be the 

 realized tide-mill, and however convenient and non- 

 wasteful the acumulator — whether Faure's electric accu- 

 mulator, or other accumulators of energy hitherto in- 

 vented, or to be invented,- -which might be used to store 

 up the energy yielded by the tide mill during its short 

 harvests about the times of high and low water, and to 

 give it out when wanted at other times of six hours. 

 There may, however, be a dozen places possible in the 

 world where it could be advantageous to build a sea- 

 wall across the mouth of a natural basin or estuary, and 

 to utilize the tidal energy of filling it and emptying it by 

 means of sluices and water-wheels. But if so much could 

 be done, it would in many cases take only a little more to 

 keep the water out altogether, and make fertile land of 

 the whole basin. Thus we are led up to the interest- 



