ON STATICS. 101 



more usually draw or pull in an ascending direction, so that our whole 

 muscular force may be exerted without any limit of this kind.* 



It happens, however, very frequently, that we have occasion for 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 hand and fingers gives the 

 human species a great superiority over all other animals, although by no 

 means, as some authors have supposed, an advantage equivalent to that of 

 the higher perfection of the intellectual powers. It is true, as we may 

 observe in the manufactories of this country, that machinery has been 

 invented by which a power 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 difficult 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 which 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 convenient unit for the 

 comparison of moving powers of all kinds. It may be most easily remem- 

 bered 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 day. The 

 actual velocity of the man's motion must vary according to the mode in 

 which his force is applied ; 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. Desagulierst 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 support of his estimate. Professor 

 Robison, however, mentions a hydraulic machine 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 which the water must 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 Mr. Bucha- 

 nan's J experiments, an action like that of ringing bells produced an effect 

 about one third greater than turning a winch, and the action of rowing, an 

 effect four ninths greater ; but it does not appear that these experiments 

 were continued for a whole day ; and the greatest number of observations 

 make the daily performance of workmen considerably less. It is indeed 

 seldom that the muscles employed in progressive motion are so much 

 exerted as in the arrangement described by Professor Robison. A Chinese, 

 in the operation called sculling, is said to beat a European at his oar. 

 For a short time a much greater effect than this may be produced by a 



* See Mairan on the position of the legs in walking, Hist, et Mem. de Par. 1721, 

 t Desaguliers, vol. i. pp. 254, 255. J Repertory of Arts, xv. 319. 



