102 LECTURE XII. 



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 perhaps 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 employed, 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 incon- 

 siderable. 



The action of carrying a load horizontally requires an exertion of a 

 different kind, and admits of no direct comparison with the application of 

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

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

 confined within moderate limits. A strong porter can carry 200 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 t 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 imme- 

 diate force is something greater, but it cannot support the labour of more 

 than 8 hours a day, when drawing with a force of 200 pounds, or of 6 

 hours when with a force of 240, walking two miles and a half an hour. It 

 is generally supposed that in drawing up a steep ascent a horse is only 

 equivalent to 3 or 4 men, and the employment of horses in walking wheels, 

 where the action is similar to that of ascending a hill, has for this reason been 

 condemned. For men, on the contrary, an ascent of any kind appears to 

 afford a favourable mode of exertion. But, perhaps, the weight of the 

 carriage, and of the horse itself, has not always been sufficiently considered 

 in the comparison. The strength of a mule is equal to that of three or four 

 men. The expense of keeping a horse is in general about twice or three 

 times as great as the hire of a day labourer ; so that the force of horses 

 may be reckoned about half as expensive as that of men. The horse 

 Childers is said, although, perhaps, without sufficient authority, to have 

 run an English mile in a single minute ; his velocity must in this case 

 have been 88 feet in a second, which would have been sufficient to carry 



* See Amontons, Hist, et Mem. de 1'Acad. 1703. 



t On the Daily Labour of Men, Nich. Jour. iii. 416. 



