APPLICATION OF FOBCE. 89 



labor, expended in raising a pound weight one foot tigh, 

 in opposition to gravity. 



How is the ef- ^^^' The sffcct produced by a moving power 

 fng powe"es'- ^^ always expressed by a certain weight raised 

 pressed? ^ Certain height. 



To find, therefore, the effect of a moving power, or to find the power ex- 

 pended in performing a certain work, we have the following rule : — 



How may the ^^l. Multiply the weight of the body moved 

 Sd'^in'w^rbe' i° pounds by the vertical space through which 

 *~*'^^«'^' it is moved. 



Thus, for example, if a horse draw a loaded wagon, with a fonce by which 

 the traces are stretched to as great a degree as if 200 pounds v> re suspended 

 vertically fi-om them, and if the horse thus acting draws the wagon over a 

 space of 100 feet, the mechanical effect produced is said to be 200 pounds 

 raised 100 feet; or, what is the same thing, 20,000 pounds raised 1 foot. 

 "When a horse draws a carriage, the work he performs is expended in over- 

 coming the resistance of friction of the road which opposes the motion of tho 

 carriage ; but friction increases and diminishes as the weight of the load in- 

 creases or diminishes. The work performed will, therefore, be estimated by 

 multiplying the total resistance of friction, as expressed in pounds, by the 

 space over which the carriage is moved. 



J.. , The following examples will illustrate how we are enabled, 



manner of esti- by the above rules, to calculate the amount of power required 

 mating power? ^ perform a certain amount of work: — Suppose we wish to 

 know the amount of horse-power required to lift 224 pounds of coal from tho 

 bottom of a mine 600 feet deep. The weight, 224, multiplied into space 

 moved over, 600 feet, equals 134,400, the amount of work to be performed 

 each minute; a horse-power equals 33,000 pounds raised 1 foot per minute: 

 therefore, 134,400-^-33,000=4.07, horse-power required. If we wish to per- 

 form the same work by a steam-engine, we would order an engine of 4.07 

 horse-power, and the engine-builder, knowing the dimensions of the parts of 

 an engine essential to give one horse-power, can build an engine capable of 

 performing the requisite work. 



Again. Suppose a locomotive to move a train of cars, on a level, at tho 

 rate of 30 miles per hour, the whole weighing 25 tons, with a constant re- 

 sistance from friction of 200 pounds, what is the horse-power of the engine 7 

 30 miles per hour equals 2,640 feet per minute ; this space multiplied by 200 

 pounds, the resistance to be overcome, equals 528,000, the work to be done 

 every minute; which, divided by 33,000 (one horse-power), equals 16, tho 

 horse-power of the locomotive. 



What is a Dy- 182. Au instrument for measuring the rela- 

 namometer? ^j^.g strength of meu and animals, and also of 

 the force exerted by machinery, is called a Dynamometer. 



