SCIENTIFIC RECREATIONS. 



inches. We will prove this by illustration. Take a bent tube and fill it with 

 mercury ; the liquid will stand equally high in both arms, in consequence 

 of the ratio of equilibrium in fluids, of which we shall read more when we come 



f A* N to consider Water. So the two columns of mercury 



are in equilibrium. (See A.) Now stop the arm a 

 with a cork, and take out half the mercury. It will 

 remain in one arm only. Remove the cork, and the 

 fluid will fall in both arms, and remain in equilibrio. 

 If a long bent glass tube be used, the arms being 

 thirty-six inches high, the mercury will fall to a 

 point c, which measures 29*9 inches from the bot- 

 Fi g . 4 8.-Air Pressure. tom< jf ^g tu b e k e a square inch in bore, we 



have 29-9 cubic inches of mercury, weighing 14! Ibs., balancing a column 

 of air one square inch thick and as high as the atmosphere. So the mercury 

 and the column of air must weigh the same. Thus every square inch on 

 the earth supports a weight of (nearly) 15 Ibs (figs. 48 and 50). 



Fig. 49. The Barometer. 



The barometer invented by Pascal, working on the investigations of 

 Torricelli, is a very simple and useful instrument. Fill a tube with mercury 

 from which all moisture has been expelled, and turn it over in a dish. 



