276 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



water as 13-59 to 1, the height of the mercury column, 

 which would balance the atmospheric pressure, is 

 ]3'59 times less than 34 feet, that is nearly 30 inches 

 of mercury, in French measure exactly 760 milli- 

 metres. This method of measuring the weight of the 

 atmosphere is due to Torricelli, a pupil of Galileo. 



The Torricellian experiment is performed by 

 taking a glass tube about 3 feet long, and a quarter of 

 an inch internal diameter, closed at one end. The 

 tube is completely filled with mercury ; the open end 

 is then closed by the thumb, the tube inverted in a 

 vessel of mercury, and secured in the 

 vertical position. On withdrawing the 

 thumb, the mercury sinks a short distance 

 in the tube, leaving a vacuous space above, 

 and after a few oscillations remains at 

 a certain height, which is determined by 

 the atmospheric pressure at the place. 

 The tube, therefore, becomes a measurer 

 of the pressure at the place, a BAROMETER. 

 Should the pressure increase, the mercury 

 rise in the tube, there being no ail- 

 above to hinder its ascent ; if the pressure 

 Fig. 119. Tor- diminishes, the mercury column will di- 

 periiaent. minish in height. At the sea level the 

 height will be 760 millimetres of mercury, 

 and in proportion as we ascend in the atmosphere 

 the mercury column becomes lower. 



It has to be noted, however, that besides the 

 height of the mercury column the temperature at the 

 time of observation must be taken into account. For 

 the density of the mercury will vary with the tem- 

 perature, diminishing with increased temperature and 

 increasing with diminished temperature. At the 

 same place, therefore, the column will stand higher 

 with a high than with a low temperature, though the 

 pressure does not vary. Accordingly the standard 



