MECHANICAL PARADOXES. 



about a hundred times as forcibly as the 

 water presses it downwards. No wonder the 

 water does not force the card away and fall 

 out ! When an accident does happen, and the 

 water falls, it is simply because in reality 

 the card has moved sideways, or twisted, and 

 so failed to distribute the atmospheric pres- 

 sure evenly. The sole duty of the paper or 

 card is to secure this even distribution of 

 pressure. 



A narrow tube, such as B, filled with water, 

 will hold it without any paper beneath it, if 

 the upper end be simply stopped by pressing 

 the finger over it. Its section area is too 

 small to let the pressure be unevenly distri- 

 buted, so that no bubbles of air can pass up- 

 wards ; and the atmospheric pressure easily 

 holds up the water. 



If two tumblers be filled under water, say in 

 the bath, and while still under water be placed 

 end to end, as at C, and have their joint wrapped 

 with a strip of paper, they may then be placed 

 vertically one over the other, as shown, and 

 lifted out of the water. The whole arrange- 

 ment can then be held by the upper tumbler 

 if care is taken to keep them vertical, and the 

 lower tumbler, with the water contained in 

 both, will be held up by the atmospheric 

 pressure beneath it. 



For this experiment it is well to take bed- 

 room tumblers, with perpendicular sides, and 



86 



