( '3 ) 49 



ing its weight to be 1,000 pounds and its velocity 1,100 

 feet per second, would give ; it is 902,797 tons ! 12 Now, 

 if it were possible to convert the whole of this enormous 

 mechanical power into heat, to how much would it cor- 

 respond ? This question may be answered by the aid 

 of the mechanical equivalent of heat ; here is the cal- 

 culation, from which we see that when 17 gallons of 

 ice-cold water are heated to the boiling point, as much 

 energy is communicated as is contained in the death- 

 dealing missile at its highest velocity. 13 Again, if we take 

 the impact of a larger cannon-ball, our earth, which is 

 whirling through space with a velocity of 19 miles a 

 second, we find it to be 98,416,136,000,000,000,000,000,- 

 000,000,000 tons I 1 * Were this energy all converted into 

 heat, it would equal that produced by the combustion 

 of 14 earths of solid coal. 1 * 



The conversion of heat into motion, however, as al- 

 ready stated, is not as perfect. The best steam-engines 

 economize only one-twentieth of the heat of the fuel. 16 

 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 remain- 

 ing 30 tons only being converted into work. 



One other quantitative determination of force has 

 also been made. Prof. Julius Thomsen, of Copenhagen, 

 has fixed experimentally the mechanical equivalent of 

 light. 1 ? He finds that the energy of the light of a sper- 

 maceti candle burning 126)4 grains per hour, is equal 

 in mechanical value to 13*1 foot-pounds per minute. 

 The same conclusion has been reached by Mr. Farmer, 

 of Boston, from different data. 18 



If we pass from the^ actual physical energies or mo- 



