$% WELLS'S NATURAL PHILOSOPHY. 



Define Power, 19^- ^^ machinery, we designate the mov- 

 wor^ng Point, ^^g foFce as thc Power ; the resistance to be 

 ^cwK *" overcome, whatever may be its nature, as the 

 Weight ; and the part of the machine im- 

 mediately applied to the resistance to be overcome, as the 

 Working Point. 



What ii the 191. The great general advantage that we 

 S.'ivHntat4"^''of obtain from machinery is, that it enables us 

 machinery r jq exchaugc time and space for power. 



Thus, if a man could raise to a certain height two hundred pounds in one 

 minute, with the utmost exertion of his strengtli, no arrangement of machinery 

 could enable him unaided to raise 2,000 pounds in the same time. If he de- 

 sired to elevate this weight, he would be obliged to divide it into ten equal 

 parts, and raise each part separately, consuming ten times the time required 

 for lifting 200 pounds. The application of machinery would enable him to 

 raise the whole mass at once, but would not decrease the time occupied in 

 doing it, which would still be ten minutes. 



Again. A boy who can not exert a force of fifty pounds may, by means 

 of a claw-hammer, draw out a nail which would support the weight of half a 

 ton. It may seem that the use of the hammer in this case creates power, 

 but it does not, since the hand of the boy is required to move through per- 

 haps one foot of space to make the nail rise one quarter of an inch. But it baa 

 been already shown that the force of a small body moving with great velocity 

 may equal the force of a large body with a slight velocity. On the same prin- 

 ciple, the small weight, or power, exerted by the boy on the end of the ham- 

 mer handle, moving through a large space with an increased velocity, ac- 

 quires sufficient momentum to overcome the great resistance of tlie nail. 



In both of these examples space and time are exchanged for power. 



„ . .. 192. The mechanical force, or momentum, of a body, is as- 



How IS the me- ' ' "' 



chanical effect certained by multiplying its weight by the space through 

 termined" ^''' '"'li'ch it moves in a given time, that is to say, by its velocity. 

 The mechanical force, or momentum, of a power may also be 

 found, by multiplying the power, or its equivalent weight, by its velocity. 



What is the 193. The power, multiplied by the space 

 {rrium*^ of "all through which it moves in a vertical direction, 

 inaciiinesf jg equal to thc weight multiplied by the space 

 through which it moves in a vertical direction. 



This is the general law which determines the equilibrium of all machines. 



„ , 194. The power will overcome the resistance 



Under what ^ -"^ . -n , i 



conditions wiu of thc wcijirht, and motion will take place in a 



motion take ii i • • n 



place in » ma- machmc, when the product arising from the 



chine t ^ , ® . 



power multiplied by the space through which 



