19S LECTURE XII. 



whole day ; and the greatest number of observations make the daily per- 

 formance of workmen considerably less. It is indeed seldom that the 

 muscles employed in progressive motion arc so much exerted as in the ar- 

 rangement described by Professor Robison. A Chinese, in the operation call- 

 ed sculling, is said to beat a European at his oar. 



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

 exertion: thus a man weighing above 160 pounds can ascend by means of 

 steps, at the rate of more than three feet in a second, for a quarter, or per- 

 haps half a minute; and this is an effort five times as great as that which can 

 be continued for a day. Usually, however, where the hands are chiefly em- 

 ployed, whether in turning a winch, or in pumping, it is only possible to exert 

 a double, or at most a triple action, for a minute or two : thus, although a 

 machine may only enable a man to raise a hogshead of water in a minute to the 

 height of ten feet for a whole day, yet it is easy to work it so rapidly for a 

 single minute as to raise double the quantity, or to raise a single hogshead to 

 a height of twenty feet. The whole exertion of force must be a little greater 

 than that which is thus estimated, because a certain degree of superfluous 

 momentum must be generated in removing weights from one situation to 

 another: but this loss is usually inconsiderable. 



The action of carryijig a load horizontally requires an exertion of a differ- 

 ent kind, and admits of no direct comparison with the application of a 

 constant force to overcome the gravitation of a weight, or any other im- 

 mediate resistance. The work of a labourer thus employed is however 

 confined within moderate limits. A strong porter can carry §00 pounds at 

 ■the rate of three miles an hour; and, for a short distance, even 300 pounds: 

 a chairman carries 150 pounds, and walks four miles an hour; and in Turkey 

 it is said that there are porters, who, by stooping forwards, and placing the 

 weight very low on their backs, are enabled to carry from 700 to 900 pounds. 

 The subjects of Mr. Coulomb's experiments appear to have been either weaker, 

 or more inactive, than the generality of porters in this country: he calculates 

 that the most advantageous load for a man of common strength is about a 

 hundred weight; or, if he is to return without a burden, 135 pounds. 



The daily work of a horse is equal to that of five or six men: its immediate 



