26 ELEMENTARY AGRICULTURAL CHEMISTRY 



the atmosphere. This simple form of barometer is the best, 

 and, with proper means for accurately measuring the vertical 

 distance between the two mercury surfaces, is the most 

 accurate. The ordinary weather-glass^ or wheel barometer* 

 though it can be made very sensitive — i.e., capable of showing 

 even a small change in pressure — has no claims to accuracy. 



The aneroid barometer depends for its action upon the elas- 

 ticity of a steel box which has been completely emptied of air. 

 The pressure of the air causes the lid of the box to collapse 

 partially, and the amount of the indentation will be greater or 

 less as the pressure varies. The motion of the lid is indicated 

 by the rotation of a pointer moved by a chain attached to the 

 centre of the lid, and controlled by a light spring. 



As the height of the barometer measures the pressure, 

 which depends upon the weight of the air above, it is obvious 

 that the pressure, and therefore the barometric reading, will 

 diminish as the altitude above the sea-level increases. Hence 

 it is possible to measure this by means of the barometer. 

 The relation between the difference in the vertical heights of 

 the two stations and the difference in the barometric readings 

 depends upon several conditions. Roughly speaking, it is found 

 that near sen level for an ascent of about 900 feet the barometer 

 falls one inch, while from a height of 5000 feet an ascent of 

 about 1100 feet is accompanied by a fall of an inch in baro- 

 metric pressure. 



Air, like other gases, is altered in volume by changes in 

 temperature or pressure. If the former remain constant tho 

 volume of a given quantity of gas is inversely proportional to 

 the pressure. This relation was discovered by Boyle in 1661, 

 and is nearly true for all gases between small limits of pressure. 



If the pressure be constant the volume of gas varies 

 directly as the temperature, measured from the absolute zero of 

 temperature. The absolute zero is apparently at a point 

 273° C, or 491° F., below the melting-point of ice— -i.e., 

 - 273° C, or - 459° F. By the application of this generalisa- 

 tion, the calculation of the volume which a known quantity of a 



