EFFORT MADE IN WALKING ON LEVEL GllOUND. 517 



on foot by postmen namely, twenty miles a-day throughout 

 the year, with rest on Sundays. In this standard chosen by 

 Professor Playfair, it is difficult to make a comparison between 

 the weight of the body carried along horizontally by the 

 locomotive organs for a given distance, and the mechanical 

 effort of raising a pound weight to a foot high. It is com- 

 monly allowed that five or six times as great a weight can be 

 carried horizontally as can be drawn along the ground with a 

 like effort. Again, the body is moved along a level surface by 

 the motions of the limbs with a facility like that given by 

 wheels to a burden. It is apparently from such data as these 

 that Professor Playfair assumes the resistance of a man 150 

 Ib. weight in walking to be no more than ^ or 7.5 lb., so that 

 the number 15,840,000, or that brought out by multiplying 

 the number of feet in twenty miles by the number of pounds 

 in the body, being divided by 20, affords his number of foot- 

 pounds, 792,000. There is no other fault to be found with 

 this number as the standard of a man's daily moderate work, 

 than the difficulty of finding out how it is arrived at. Even 

 if this relation be correct, as respects the two cases of a 

 man walking unloaded and a man raising a load equal to 

 his own weight, it seems at first sight wholly inapplicable 

 to the comparison between the effort of a man walking with 

 a load, and one raising perpendicularly a load equal to that 

 load and his own weight. And yet the rule is probably 

 quite correct in every case in which the load is not greater 

 than can be carried for the period of five or six hours. Thus, 

 to take one of Dr Young's examples, " a strong porter," he 

 says, " can carry 200 lb. at the rate of three miles an hour, 

 and for a short distance even 300 lb." In the first case, if the 

 effort can be continued for six hours a-day, as is not unlikely, 

 with interposed periods of rest, then the sum of the successive 

 efforts will afford one equal to the carrying one lb. horizontally 



