BY EXERCISE. 881 



are nourished, that the power of vision is dimi- 

 nished by exposure of the eyes to a dazzling 

 light, for the same reason that the brain is ex- 

 hausted by intense thinking, and the muscles by 

 violent exercise. Hence it is, that a daily supply 

 of food is required to restore the waste of the 

 solids ; and that the amount required to maintain 

 the standard weight of the body, is always in 

 proportion to the degree of exertion. This was 

 strikingly illustrated in the case of Captain Bar- 

 clay, who, when performing his great feat of 

 walking 1000 miles in 1000 successive hours, 

 consumed daily, from five to six pounds of animal 

 food, with a proportional quantity of bread, vege- 

 tables, wine, ale, porter, and tea. (Lond. Quar- 

 terly Rev. vol. 65, p. 322.) It has also been 

 ascertained, that without a large addition to the 

 usual allowance of prisoners employed in the 

 treadmills of England, they lose about one pound 

 in weight every week, and become greatly ema- 

 ciated in a few months, that they are extremely 

 liable to severe colds, rheumatism, bowel com- 

 plaints, scurvy, and other maladies arising from 

 debility. 



It is because the substance and vital energy 

 of the body are expended by exertion, that the 

 growth of all young animals is retarded by pre- 

 mature labour. Hence also it is, that the mean 

 duration of life among the working classes in 

 England, does not exceed from 20 to 30 years ; 



