422 THE CANADIAN NATURALIST. [DeC. 



cover of whicli passes a shaft, carrying upon its lower end a set of 

 paddles, immersed in water within the box, and upon its upper 

 portion a drum on which are wound two cords, which, passing in 

 opposite directions,and over pulleys, and are attached to known 

 weights. The temperature of the water within the box being 

 carefully noted, the weights are then allowed to fall a certain num- 

 ber of times, of course in their fall turning the paddles agains* 

 the friction of the liquid. At the close of the experiment the 

 water is found to be warmer than before. And bv measurino; the 

 amount of this rise in temperature, knowing the distance through 

 which the weights have fallen, it is easy to calculate the quantity 

 of heat which corresponds to a given amount of motion. In this 

 way, and as a mean of a large number of experiments, Mr. Joule 

 found that the amount of mass-motion in a body weighing one 

 pound, which had fallen from a height of 772 feet, was exactly equal 

 to the molecular motion which must be added to a pound of water, 

 in order to heat it one degree Fahrenheit. If we call the actual 

 energy of a body weighing one pound which has fallen one foot, a 

 foot-pound, then we may speak of the mechanical equivalent of 

 heat as being 772 foot-pounds. 



The significance and value of this numerical constant will ap- 

 pear more clearly if we apply it to the solution of one or two 

 simple 231'oblems. During the recent war two immense iron guns 

 were cast in Pittsburgh, whose weight was nearly 112,000 pounds 

 each, and which had a caliber of 20 inches.'^ Upon this diagram 

 is a calculation of the effective blow which the solid shot of such 

 a gun, assuming its weight to be 1,000 pounds and its velocity 

 1,100 feet per second, would give ; it is 902,797 tousj^ Now, if 

 it were possible to convert the whole of this enormous mechanical 

 power into heat, to how much would it correspond ? This question 

 maybe answered by the aid of the mechanical equivalent of heat 5 

 here is the calculation, 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.'^ 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.^* Were this energy all converted into heat, 

 it would equal that produced by the combustion of 14 earths of 

 solid coal.'^ 



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



