ON STATICS. 131 



It happens, however, very frequerttly, that we have occasion foi' motions of 

 such a nature as to be more conveniently performed by the hands and arms 

 than by the action of walking or running; and where delicacy is required 

 rather than strength, the form of the hantl and lingers gi\'es the human spe- 

 cies a great superiority overall other auinmls, although by no means, as some 

 authors have supposed, an advantage equivalent to that of the higher perfec- 

 tion of the intellectual powers. It is true, as we may observe in the manu- 

 factories of this country, that machinery has been invented by which a jjower 

 of any kind may be converted to purposes seemingly the most intricate and 

 refined ; and after all that has been done by a Watt and an Arkwright, it is diffi- 

 cult to determine a positive limit to the ingenuity of mechanical invention. 



It is necessary to consider, in examining the different sources of motion, 

 not only the immediate magnitude of the forces which they produce, but also 

 the velocity with whieh they are capable of acting, and the time for which 

 that action can be continued. The daily work of a labouring man, of middle 

 age, and in good health, will serve as a consenient unit for the comparison 

 of moving powers of all kinds. It may be most easily rememl)ered in this 

 form: a man can raise a weight of 10 pounds to the height of 10 feet in a 

 second, and can continue this labour for 10 hours a da\-. The actual velocity 

 of the man's motion must vary according to the mode in which his force is ap- 

 plied ; but we suppose that velocity to be such as to give the greatest effect 

 under the circumstances of the machine. This is a moderate estimate of the 

 work of a labourer, without any deduction for friction. Desaguliers states 

 the performance of a man working at a winch, with the assistance of a fly, as 

 considerably greater, but he does not allege any correct experiments in sup- 

 port of his estimate. Professor Robison, however, mentions a hydraulic ma- 

 chine in which the effect was actually more than one tenth greater, without 

 making any allowance for friction; so that it is probable, considering the loss 

 both from friction and from the momentum with wliich the water nuist have 

 been disengaged, that the immediate performance was at least one third more 

 than this unit: the machine was worked by a light'man carrying a weight, and 

 walking backwards and forwards on a lever. According to ]\fr. Buchanan's ex-. 

 perinients,an action like that of ringing bells produced an effect about one third 

 greater than turning a winch, and the actir)n of rowing, an effect four ninths 

 greater; but it does not appear that these experiments were continued for a 



