PROPERTIES OF GASES 



65 



maintained between the column of mercury in the tube and a column 

 of air outside it, pressing down upon the mercury in the basin. 



A column of mercury will be supported in the tube by the pressure 

 of the atmosphere. The distance between the top of the column and 

 the surface of the mercury in 

 the cup will be about 30 inches, 

 or 76 cm., when the tube is 

 vertical (Fig. 52, 6). If the 

 tube is inclined so that the 

 closed end of it is less than this 

 height above the mercury in 

 the cup (Fig. 52, c) the mercury 

 fills it completely ; and if the 

 tube is less than 30 inches long, 

 it is always filled by the mer- 

 cury whether it is inclined or 

 not (Fig. 52, a). On an aver- 

 age the atmosphere at sea-level 

 will balance a column of mer- 

 cury 30 inches in height. No 

 matter if the closed tube is 

 30 feet long, the top of the 

 mercury column will only be 

 about 30 inches above the 

 level of the mercury in the 

 basin. 



The empty space above the 

 column of mercury in the tube 

 is often referred to as the 

 Torricellian vacuum. 



FIG. 52. The atmosphere at sea-level will 

 support a column of mercury up to 30 inches, 

 or 76 cm. in length. 



The aneroid barometer. Owing to its necessary size and weight, 

 the mercury barometer is not a portable instrument. The aneroid 

 barometer has been designed to get over this 

 difficulty. It consists essentially of a small 

 box closed on top with a thin sheet of corru- 

 gated metal, and partially exhausted of air 

 (Fig. 52a). This metal top is very sensitive to 

 variations of pressure, and by means of levers 

 the movement of the metal is magnified and 

 transmitted to a circular dial, where a hand 

 or pointer is rotated. The dial is graduated to read the same as a 

 mercury barometer, by comparison with one of the latter, 

 j.o.s. E 



FIG. 52et. Section of an 

 aneroid barometer. 



