112 ELEMENTARY GENERAL SCIENCE CHAP. 



light as mercury, the column of watef which could be supported 

 would be 30 x 13 '6 = 408 inches = 34 feet, which would not be 

 a convenient length for a barometer. The length of the column 

 of glycerine which can be similarly supported is 27 feet. But 

 in the case of lighter liquids like these, any small variation in 

 the weight of the atmosphere is accompanied with a much 

 greater alteration in the level of the column of liquid, and in 

 consequence it is possible to measure such variations with much 

 greater accuracy. For this reason barometers are sometimes 

 made of glycerine. There is one in the Western Galleries of 

 the South Kensington Museum, which the student could see at 

 any time. 



Pressure of the Atmosphere at Different Altitudes. The 

 atmosphere being a material substance, the longer the column of 

 it there is above the barometer, the greater will be the weight of 

 that column, and the greater the pressure it will exert upon the 

 mercury in the barometer. Hence, as we ascend through the 

 atmosphere with a barometer, we reduce the amount of air above 

 it pressing down upon it, and in consequence the column of 

 mercury the air is able to support will be less and less as we 

 ascend. On the contrary, if we can descend from any position, 

 e.g., down the shaft of a mine, the mercury column will be 

 pushed higher and higher as we gradually increase the length of 

 the column of air above it. Since the height of the column of 

 mercury varies thus with the position of the barometer, it is 

 clear that the variation in its readings supplies a ready means of 

 ascertaining the height of the place of observation above the 

 sea-level, provided we know the rate at which the height of the 

 barometer varies with an alteration in the altitude of the place. 

 The rule which expresses this relation is not a simple one, but 

 for small elevations it is said that a rise or fall of one inch in the 

 height of the barometer corresponds to an alteration of 900 feet 

 in the altitude of the barometer. 



Relation between Volume and Pressure of a Gas. Boyle's 

 Law. Before we can properly understand how and why the 

 density of the atmosphere varies, it is necessary to become 

 acquainted with the rule expressing the relation between the 

 volume and pressure of a gas. A convenient and efficient 

 arrangement for determining this relation is represented in Fig. 

 50, and it has the merit of being easily constructed with the 

 materials which should be found in every physical laboratory. 

 A burette, A, is fixed to an upright stand by means of wire or 



