DISCUSSION AND COBBESBONDENCE. 



209 



so far as the stone is concerned no work 

 is done. So a man may hold a heavy 

 weight in his hand or on his shoulder, 

 sustaining it with considerable effort 

 against the force of gravity, and yet no 

 work is done on the stone so long as it 

 is not raised to a higher level. If the 

 stone is carried in a horizontal plane, 

 no work is done on the stone; while if 

 it is carried down hill or lowered verti- 

 cally, negative work is done on the 

 stone. That is, since the stone possesses 

 less potential energy at the foot of the 

 hill than at the top (the difference being 

 equal to the weight of the stone multi- 

 plied by the difference of altitude), the 

 stone has lost energy, and this energy 

 lost by the stone has been communi- 

 cated to the man, who has had work 

 done upon him by the stone, albeit he 

 may have lugged it down the hill or 

 lowered it from an elevated position 

 with considerable effort. 



When a car is propelled by an elec- 

 tric motor deriving its current from a 

 storage battery carried on board the car, 

 the energy of the car consists of three 

 parts: (A) Mechanical potential energy 

 due to the mass of the car being at 

 some elevation above the surface of the 

 earth. (B) Kinetic energy, due to the 

 motion of the car as a whole and of its 

 parts with respect to one another and 

 the heat of the car. (C) Chemical po- 

 tential energy stored up in the battery. 

 When the car is running up grade, en- 

 ergy is being expended not only in over- 

 coming friction, but also in lifting the 

 car against the force of gravity. In 

 doing this, energy is transferred from C 

 to A. When the car descends again to 

 its former level the energy stored up in 

 A is given up, less energy is therefore 

 required from the battery to propel the 

 car, and the battery is accordingly in 

 so much spared. If the grade be steep, 

 the motor may actually be driven as a 

 dynamo, and the current which is there- 

 by generated may be stored up in the 

 battery. In this case energy is trans- 

 ferred from A to C, and at the bottom 

 of the hill the energy C may be greater 

 than that at the top. The battery has 



VOL. LVIII.— 14 



done negative work on the car coming 

 down the hill: that is, the car has done 

 work on the battery and stored up en- 

 ergy. 



The same considerations apply to the 

 animal body. If a man carries himself 

 up a hill, he is doing work upon his 

 body in so elevating it against the force 

 of gravity, and if he weighs 150 pounds 

 and ascends an altitude of 10,000 feet, 

 he has done 1,500,000 foot-pounds of 

 work upon his body. This represents 

 the quantity of energy which has been 

 transferred from his tissues to his body 

 as a mass; from chemical potential en- 

 ergy to mechanical potential energy. 

 The tissues correspond to the storage 

 battery, the muscles to the motor and 

 the man's weight to that of the car. So 

 when the man walks down the moun- 

 tain again he does negative work, low- 

 ering his body (like lowering the car), 

 involving the transfer of potential en- 

 ergy from his body as a mass to his 

 tissues. Just what form the energy 

 takes as it is so transferred is not alto- 

 gether clear, but the distinction between 

 the potential energy of the body as a 

 mass, due to its elevation above the sur- 

 face of the earth, and the potential and 

 kinetic energy resident in the tissues of 

 the body, is one of fundamental impor- 

 tance and should be kept clearly in view. 



We may consider the man to be a 

 complex machine, weighing, say, 150 

 pounds and having a quantity of poten- 

 tial and kinetic energy stored up within 

 his body, which store of energy is 

 drawn upon whenever external work is 

 to be done, and which, besides, is being 

 constantly expended in keeping the body 

 warm and performing the internal work 

 of the body. The energy of the body, 

 like that of the electric car, then, con- 

 sists of three portions, viz.: (A) Me- 

 chanical potential energy of the body 

 as a whole, due to its position with re- 

 spect to the earth. This is zero when it 

 is at the earth's surface, or say the sea 

 level, and increases as it rises above the 

 sea level. (B) Kinetic energy, due to 

 the heat of the body and to the motion 

 of the body as a whole and of its several 



