I 

 ON STATICS. i03 



him on an inclined plane without friction, or in a very long sling, to the 

 perpendicular height of 120 feet.* 



A large windmill, on which Mr. Coulombt made many experiments, was 

 capable, on an average, of working eight hours a day ; its whole perform- 

 ance was equivalent to our estimate of the daily labour of 34 men ; 25 

 square feet of the sails doing the work of one labourer. The expense of 

 the machinery, with its repairs, would probably amount to less than half 

 the expense of a number of horses capable of exerting the same force. 

 Where a stream of water can be procured, its force is generally more con- 

 venient, because more regular, than that of the wind. 



A steam engine of the best construction, with a thirty inch cylinder has 

 the force of forty horses ; and, since it acts without intermission, will per- 

 form the work of 120 horses, or of 600 men, each square inch of the 

 piston being nearly equivalent to a labourer. According to Mr. Boulton, 

 the consumption of a bushel, or 84 pounds of coals, will raise 48,000 cubic 

 feet of water 10 feet high, which is equivalent to the daily labour of 8^ 

 men, or perhaps more : the value of this quantity of coals is seldom more 

 than that of the work of a single labourer for a day ; but the expense of 

 the machinery generally renders a steam engine somewhat more than half 

 as expensive as the number of horses for which it is substituted. Accord- 

 ing to other accounts, a 24 inch cylinder, being equivalent to about 72 

 horses, requires only a chaldron of coals in a day, each bushel doing the 

 work of ten men. 



The force of gunpowder is employed with advantage where a very 

 powerful action is required for a short space, as in dividing rocks, or in 

 generating a great velocity in a projectile. As a source of momentum or 

 energy only, this power is by no means economical, the daily labour of a 

 man being equivalent to the effect of about 40 pounds of powder ; but the 

 advantage of artillery consists in having the force communicated by means 

 of an elastic fluid extremely rare, which is capable of generating a very 

 great velocity in the ball only, without any waste of power in producing a 

 useless momentum in any other substance. 



The comparative force of different kinds of gunpowder is determined by 

 an eprouvette or powder proof; the effect is measured by the angular 

 motion of a little wheel, a projecting part of which is impelled by the 

 explosion of a small quantity of the powder, while the friction of a spring 

 or a weight creates a resistance which may be varied if it be required. The 

 absolute force of a given quantity of powder may be ascertained either by 

 suspending a cannon as a pendulum, and measuring its angular recoil ; or 

 by shooting into a large block, and finding the velocity which is imparted 

 to it by the ball.^ 



For measuring very small attractive or repulsive forces, with great accu- 



* Messrs. Boulton and Watt caused experiments to be made with the strong 

 horses used in the breweries in London, and from the result of their trials, they 

 assigned 33,0001bs. raised one foot per minute, as the value of a horse's power. This 

 is the unit of engine power now universally adopted. Lardner on the Steam Engine, 

 1840, p. 288. 



f Hist, et Mem. 1781, p. 65. Theorie des Machines Simples, 4to, 1821. 



j See the latter part of Lect. IV. 



