1J4 LECTURE Xir. 



cylinder, being equivalent to about 1% horses, requires ouly a chaldron of 

 coals in a day, each bushel doing the w6rk 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 ex- 

 tremely 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 l>y 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 ac- 

 curacy, the most convenient test is furnished by the effects of twisting. An 

 arm or beam is suspended horizontally by a long wire, and the force re- 

 quired to cause the beam to make one or more revolutions being ascertained^ 

 we may divide the circle described by its extremities into as many parts as 

 we think proper, and the force required to bring the beam into any position 

 will always be proportional-, without a sensible error, to the magnitude of 

 the part of the circle intercepted between the given position, and that in 

 which the arm would naturally rest. When the force is of such a nature as 

 to be capable of producing a vibration, the body on which it acts being sus- 

 pended by the thread of a silkworm, or of a spider, we may compare its 

 magnitude Avith that of gravitation, by observing the time required for each 

 . vibration, and determining the operation of the force according to the laws 

 of pendulums. It is in this manner that the forces concerned in the effects 

 of electricity and of magnetism have been measured by Mr. Coulomb. 



