Struck is perfectly rigid) is double what would occur were the stop- 

 page to occur at the end of a corresponding advance against a uni- 

 form resistance," this result must be multiplied by two ; and we gel 

 (45 I >398'7X2) 902,797 tons as the crushing pressure of the ball un- 

 der these conditions. [The author's thanks are due to his friends 

 Pres. F. A. P. Barnard and Mr. J. J. Skinner for suggestions on 

 the relation of impact to statical pressure.] 



13 The unit of impact being that given by a body weighing one 

 pound and moving one foot a second, the impact of such a body 

 falling from a hight of 772 feet the velocity acquired being 222)4 

 feet per second (=*/~^) would be IX (2223^)2=49,408 units, the 

 equivalent in impact of one heat-unit. A cannon-ball weighing 

 1000 Ibs. and moving noo feet a second would have an impact of 

 ( i ioo) 2 X 1000= i, 2 10,000,000 units. Dividing this by 49,408, the 

 quotient is 24489 heat-units, the equivalent of the impact. The 

 specific heat of iron being '1138, this amount of heat would raise 

 the temperature of one pound of iron 215,191 F. (24,489 X '1138) or 

 of 1000 pounds of iron 215 F. 24489 pounds of water heated one 

 degree, is equal to 1363-0 pounds, or 17 gallons U. S., heated 180 

 degrees ; ;'. *?., from 32 to 212 F. 



14 Assuming the density of the earth to be 5-5, its weight would 

 be 6,500,000,000,000,000,000,000 tons, and its impact by the for- 

 mula given above would be 1,025,000,000,000,000,000,000,000,000,- 

 ooo foot-tons. Making the same supposition as in the case of our 

 cannon-ball, the final pressure would be that here stated. 



15 TYNDALL, J., Heat considered as a mode of Motion ; Air), ed., 

 p. 57, New York, 1863. 



1 6 RANKINE (The Steam-engine and other prime Movers, Lon- 

 don, 1866,) gives the efficiency of Steam-engines as from i-i5th to 

 I -20th of the heat of the fuel. 



ARMSTRONG, Sir WM., places this efficiency at i-ioth as the 

 maximum. In practice, the average result is only i-3Oth. Rep. 

 Brit. Assoc., 1863, p. liv. 



HELMHOLTZ, H. L. F., says : "The best expansive engines give 

 back as mechanical work only eighteen per cent, of the heat gen- 

 erated by the fuel." Interaction of Natural Forces, in Correlation 

 and Conservation of Forces, p. 227. 



