216 PNEUMATICS. 



thirty-three feet below the piston, a height correspond- 

 ing to the weight of the atmosphere. 



THE BAROMETER. 



On the same principle the Barometer is made. It 

 consists of a glass tube, nearly three feet long, open at 

 one end, and which is first filled with mercury, a liquid 

 nearly fourteen times heavier than water. The open 

 end is then placed downward in a cup of mercury. 

 The weight of the mercury in the tube causes it to de- 

 Fig. 180. scend until the pressure of the atmosphere on 

 the mercury in the cup preserves an equilibrium, 

 which takes place when the column in the tube 

 has fallen to about two feet and a half high, the 

 upper part of the tube being left a perfect vac- 

 uum, as no air can enter {Fig. 180). Now, as 

 the height of the column of mercury depends 

 alone upon the weight of the atmosphere, then, 

 whenever the air becomes fighter or heavier, as 

 *'^'"- it constantly does during the changes of the 

 weather, the rising or falling of the column indicates 

 these changes ; and, what is very important, it shows 

 the approaching changes of the weather several hours 

 before they actually take place. Hence it becomes a 

 valuable assistant in foretelling the weather. When 

 the mercury falls, showing that the atmosphere is be- 

 coming lighter, it indicates the approach of storms or 

 rain ; when it rises, a settled or fair sky follows. These 

 are often foreshown before there is any change in the 

 appearance of the sky. For this reason the barometer 

 is sometimes called a weather-glass. It is of the 

 greatest value to navigators at sea. Long voyages 



