66 



GENERAL SCIENCE 



to their healthful state, and this is as true of the brain and 

 nervous system as of other parts of the body. The person 

 who maintains many interests in life, and a variety of 

 activities both mental and manual, is not only likely to 

 lengthen life thereby but to add to it many years of efficiency 

 and of enjoyment in living. 



Rest is essential for both body and mind that their activi- 

 ties may be continued. The human body as a machine at 

 work does not provide energy rapidly enough, nor make 



FIG. 27. Gardening is good exercise. 



tissues fast enough, to keep up with the waste. Unlike 

 ordinary machines it is rebuilding its parts as it goes along. 

 This requires time, and favorable conditions, and a suitable 

 food supply. Sleep is needful to keep the body vigorous 

 and in a healthy state. Those hours that are spent in sound 

 refreshing sleep are not wasted. Sleep to be refreshing 

 should be free from disturbances and interruptions, and from 

 the ill-effects of an intemperate diet. 



Eight hours per day is enough sleep for people generally. 



