The Science of the Weather 775 



tube is closed, while the lower end is open and dips into a cistern 

 containing mercury. The tube is first of all filled completely 

 with mercury and the open end is carefully covered as, for 

 example, by the finger so that no mercury can escape from, nor 

 air enter, the tube; then the tube is inverted and the open end 

 immersed in the cistern of mercury. If the finger is now removed 

 the mercury in the tube will be seen to fall until the top of the 

 column stands somewhere about 30 inches above the level of the 

 mercury in the cistern, and the space that is left between the top 

 of the mercury column and the upper end of the tube is an 

 absolute vacuum. The column of mercury in the tube is pre- 

 vented from running out completely by the pressure exerted by 

 the atmosphere on the surface of the mercury in the cistern, and 

 this implies that the weight of the mercury column must be 

 exactly balanced by the weight of a column of air of equal cross- 

 section, but extending from the surface of the mercury in the 

 cistern up to the absolute limit of the earth's atmosphere. There- 

 fore, when we talk of the rising and falling of the barometer it 

 means that the length and, therefore, weight of the mercury 

 column is increasing or decreasing, and that the weight or pres- 

 sure of the counterbalancing air column is changing sympa- 

 thetically. In other words, in the barometer the pressure 

 due to the weight of mercury is equal to that exerted by the 

 atmosphere. 



Photography Used for Recording Purposes 



In the chief observatories a simple barometer is mounted in 

 an ingenious form of camera, and a continuous photographic re- 

 cord of its fluctuations is obtained upon a sheet of bromide paper 

 mounted on a cylinder, which is rotated by clockwork. The 

 record thus obtained is called a "barogram," and is amenable to 

 the most accurate measurement, its tabulated readings forming a 

 basis for statistical analysis of pressure variations. 



Another instrument used for recording pressure is the 



