248 THE PHENOMENA OF MOTION 



The restoration of force is effected, in the animal 

 body, by the transformation of the separated parts, 

 destined for the production of force, and by the ex- 

 penditure of the active vital force in causing forma- 

 tion of new parts ; and, with the restoration of the 

 separated or effete parts, the organism recovers a 

 force equal to that which has been expended. 



It is plain, that the vital force manifested, during 

 sleep, in the formation of new parts must be equal 

 to the whole sum of the moving power expended in 

 the waking state in all mechanical effects whatever, 

 plus a certain amount of force, which is required for 

 carrying on those involuntary motions which con- 

 tinue during sleep. 



From day to day, the labouring man, with suffi- 

 cient food, recovers, in seven hours' sleep, the whole 

 sum of force ; and without reckoning the force 

 necessary for the involuntary motions which may 

 be considered equal in all men, we may assume, that 

 the mechanical force available for work is directly 

 proportional to the number of hours of sleep. 



The adult man sleeps 7 hours, and wakes 17 

 hours ; consequently, if the equilibrium be restored in 

 24 hours, the mechanical effects produced in 17 hours 

 must be equal to the effects produced during 7 

 hours in the shape of formation of new parts. 



An old man sleeps only 3^ hours ; and if every 

 thing else be supposed the same as in the case of 

 the adult, he will be able, at all events, to produce 

 half of the mechanical effects produced by an adult 



