50 MECHANIC POWF.RS. 



moving power, as to obtain effects which it could 

 not produce without this modification. 



In machines composed of several of the mecha- 

 nic powers, the power will be to the weight, when 

 they are in equilibrio, in a proportion formed by 

 the multiplication of the several proportions which 

 the power bears to the weight in every separate me- 

 chanic power of which the machine consists. 



Suppose a machine, for instance, composed of 

 the axis in the wheel, and a pulley; let the axis 

 and wheel be such, that a power consisting of one- 

 sixth of the weight will balance it; and let the pul- 

 leys be such, that by means of them alone, a power 

 equal to one-fourth of the weight would support it: 

 then, by means of the axis in the wheel, and the 

 pulleys combined, a power equal to one-fourth of 

 one-sixth, that is, l-24th of the weight, will be in 

 equilibrio with it. 



In contriving machines, simplicity ought particu- 

 larly to be attended to; for a complicated machine 

 is not only more expensive, and more apt to be out 

 of order, but there is also a greater degree of 

 friction in proportion to the number of rubbing 

 parts. 



Whatever be the construction of a machine, its 

 power will always be in proportion to the velocity 

 of the power to the weight; and so that this is ob- 

 tained in the greatest degree that circumstances 

 will admit, or that are necessary, then the fewer 

 parts the better. 



It is evident, from what has been said above, 

 that the velocity of a wheel is to that of a pinion, 

 or smaller wheel which is driven by it, in pro-, 

 portion to the diameter, circumference, or number 

 of teeth in the pinion to that of the wheel. Thus, 

 if the number of teeth in a wheel be 60, and those 



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