MECHANICAL PARADOXES. 



a fire is driven up the chimney by the colder, 

 denser, heavier air of the room. Thus we get 

 a constant up-current of air from the hands 

 and face. This is so slight that in ordinary 

 circumstances it passes undetected. But if a 

 good deal of the skin is exposed when it is 

 hot and the air very cold, the up-draught 

 due to the warming and expansion of the air 

 is easily observed. 



Thus, when a bald-headed gentleman, who 

 lives a good way from the station, has made 

 himself very hot on a frosty morning by hurry- 

 ing from a late breakfast, and finds that his 

 train, being unusually punctual, has left him 

 to wait for the next, he will not unfrequently 

 pass the time during this wait by mopping up 

 the perspiration. At the moment between 

 the removal of his hat and the application of 

 his handkerchief, clouds of vapour, condensed 

 by the cold air, may be seen rising up in a 

 column from his head, clearly showing the 

 direction of the up-current which is caused 

 by the heat of his head warming the air, and 

 so making it expand and grow lighter. 



Thus it is seen that the heat from the body, 

 or a part of the body, is quite sufficient to make 

 up-currents of air capable of being employed 

 to turn a little windmill. 



Up-currents produced in this way by some 

 local source of heat, and balanced by corre- 

 sponding down-currents produced in a similar 



20 



