238 Transactions of the American Institute. 



Again, if we take the impact of a larger cannon ball, our earth, 

 which is whirling through space with a velocity of nineteen miles a 

 second, we Und it to be 98,416,136,000,000,000,000,000,000,000,000 

 tons!* "Were this energy all converted into heat, it would equal 

 that produced by the combustion of fourteen earths of solid coal.f 



The conversion of heat into motion, however, as already stated, is 

 not as perfect.. Tiie best steam engines economize only one-twentieth 

 of the heat of the fuel.:}: 



Hence if a steamship require 600 tons of coal to carry her across the 

 Atlantic, 570 tons will be expended in heating the waters of the 

 ocean, the heat of the remaining thirty tons only being converted 

 into work. 



One other quantitive determination of force has also been made. 

 Prof. Julius Thomsen, of Copenhagen, has fixed experimentally the 

 mechanical equivalent of light. § He finds that the energy of the 

 light of a spermaceti candle burning 126^ grains per hour, is equal 

 in mechanical value to 13.1 foot-pounds per minute. The same conclu- 

 sion has been reached by Mr. Farmer, of Boston, from difi^erent datju^^ 



K we pass from the actual physical energies of motions to 

 consider, for a moment, the potential energies or attractions, we 

 find, also, an intimate correlation. Since all energy not active in 

 motion is potential in attraction, it follows that in the attmctions we 

 have energy stored up for subsequent use. The sun is thus storing 

 up energy ; every minute it raises 2,000,000,000 tons of water to the 

 mean height of the clouds, 3|- miles ; and the actual energy set free 

 when this water falls is equal to 2,757,000,000,000 horse powers. | 



* Assuming the density of the earth to be 5-5, its weight would he 6,500,000,000,000,000,000,000 

 tons, and its impact, by the formula given above, would be l,025,O00.O0O.O0O,Ol'O.COO,O0O,0OO,0()O,O0;) foot- 

 ton?. Making the same supposition as in the cage of our cannon ball, the final pressure would b« 

 that here stated. 



t Tyndall, .J., Heat considered as a mode of motion. Am. ed., p. 57, New York, 1863. 



t Eankine. (The steam engine and other prime movers, London, 1866), gives the cfliciency of 

 steam engines as from l-15th to l-2()th of the heat of the fuel. 



Armstrong, Sir Wm., places this efficiency at 1-lOth as the maximum. In practice the average 

 result is only l-30th. Rep. Brit. Assoc , 1863. p. liv. 



Helmholtz, H. L. F., says "The be.-=t expansive engines give back as mechanical work only 

 eighteen per cent of the heat generated by the fuel " Interaction of Natural Forces, in Correlation 

 and Conservation of Force?, p. 227. 



§ Thomsen, Julius, Poggendorflfs Annnlcn, ex xv, 3-18. Also in abstract in Am. J. Sci., II xli, 

 296, May, 1866. 



^ American Journal of Science, II, sli, 914, March, 1866. 



I In this calculation the annual evaporation from the ocean is assumed to be about nine feet. 

 (See Dr. Buist, quoted in Maury's Phys. Geography of the Sen, New York, 1861, p. 11.) Calling 

 the water area of our globe 1.50.000.000 square miles, the total evaporation in tons per minute 

 would be that here given. Inasmuch as .'JO.OOO pounds raised on(! foot high is a horse-power, the 

 number of horse-powers necessary to raise this quantity of water 3X miles in one minute is 

 3,757,000,000,000. This amount of energy is precisely that set free again when this water falls as rain. 



