238 THE BAROMETER. 



branches of the tube, and beneath the lowest position 

 to which the mercury in the shorter branch can ever 

 fall, as shown in fig. 164 B] the height above the zero- 

 point of the mercury in each branch is read off on the 

 scale, and the difference of the two readings gives the 

 length of the mercurial column which the pressure of 

 the atmosphere supports. 



Common barometers, or so-called ' weather-glasses,' 

 are mostly unfit for accurate observations on atmo- 

 spheric pressure; the tube in most barometers of this 

 kind is too narrow, and the column is hence depressed 

 by capillarity. In fig. 165 the usual form of such 

 weather-glasses is represented, and the narrow- 

 ness of the tube indicated. By measurement of the 

 height of the column with a. metre-rule and com- 

 parison of the length found with the indications of 

 a correct barometer at the same time, this error may 

 easily be detected, and it will then be seen that the 

 scale -divisions given at the top of such barometers 

 are quite incorrect. Many barometers have, as shown 

 in fig. 165, two scales, one on each side, each being 

 divided according to a different unit. In the figure 

 the left-hand scale gives the length of the column 

 in centimetres, the right-hand scale in Paris inches. 



The empty space at the top of the tube is called 

 the Toricellian vacuum, after Toricelli, who was the 

 first to make an experiment on atmospheric pressure 

 with the barometer. To obtain a perfect Toricellian 

 vacuum (and this is absolutely indispensable in a 

 trustworthy barometer), the mercury must not only be 

 treated with particular care, before filling the tube, 



