150 THE BEASON WHY. 



w Ascribe ye strength unto God : his excellency is over Israel, and his strength 

 is in the clouds." PSAXM LVIII. 



658. Why, if a wine glass is filled with water, and a card 

 laid upon it, and the whole inverted, will the water remain in 

 the glass? 



Because the pressure of the atmosphere upon the surface of the 

 card counteracts the weight of the water. 



659. What has the card to do toith the experiment ? 



It forms a base upon which the water may rest, while the glass 

 is being inverted ; and it prevents the air from acting upon the 

 fluidity of the water, and forcing it out of the glass. 



660. Why will not leer run out of the tap of a cask until 

 a spile has been driven in at the top ? 



Because the pressure of the air upon the opening of the tap 

 counteracts the weight of the beer. But when the spile is driven 

 in, the air enters at the top, and counteracts its own pressure at 

 the bottom. 



661. Why does a cup in a pie become filled with juice ? 

 Because the heat expands the air, and drives nearly all of it out 



of the cup. When the pie is taken out of the oven, and begins to 

 cool, air cannot get into the cup again, because its edges are 

 surrounded by juice. A partial vacuum, therefore, exists within 

 the cup, and the pressure of the external air forces the juice into it. 



662. Does the cup prevent the juice from boiling over ? 

 No. So long as the heat exists, the cup remains empty ; and as 



it occupies space, the air is driven out of it, into the pie, it rather 

 tends to force the juice over the sides of the dish. It is only when 

 cooling that the juice enters the cup. 



663. Wliy can flies walk on the ceiling ? 



Because their feet are so formed that they can form a vacuum, 

 under them ; their bodies are therefore sustained in opposition t<? 

 gravitation by atmospheric pressure. 



664. How did Mr. Sands perform the feat of walking 

 across the ceiling ? 



By having large discs of wet leather attached to his feet, so that 



