chap, xxiii.] BAROMETERS. 277 



temperature is fixed at C. 5 and at this temperature 

 at the sea level the barometric height is 760 mm. 

 For higher temperatures corrections must be made. 

 Through the action of capillarity, a convex meniscus 

 (page 242) terminates the mercury column, and this, 

 modified with the height of ascent of the mercury, 

 requires also correction in very rigorous measure- 

 ments. 



At 0C., then, the pressure of a column of mer- 

 cury 760 mm. high is called the pressure of one atmo- 

 sphere. A pressure that would be equal to that exerted 

 by a column of mercury twice this height is called the 

 pressure of two atmospheres ; a pressure equal to 

 thrice the height is known as the pressure of three 

 atmospheres, and so on. There is thus a standard 

 afforded for the determination of pressures. 



Barometers. The simplest barometer is the 

 Torricellian tube fixed vertically in its vessel of 

 mercury. The mercury requires to be rid of air and 

 moisture by boiling, otherwise the Torricellian vacuum 

 would become occupied with vapour, which would 

 interfere with the rise of the mercury column. The 

 cistern barometer is a modification in which the vessel 

 containing the mercury is closed, and is supplied with 

 a bottom, movable by a screw for adjusting the level 

 of the surface. In the syphon barometer the glass 

 tube is bent, so as to have a short and a long limb. 

 The upper part of the long limb is sealed, and encloses 

 the vacuous space, the short limb takes the place of 

 the cistern, and it is open at the upper part. The 

 difference of levels in the two limbs gives the height 

 of the mercury column. The wheel barometer is just 

 the syphon barometer having a float on the surface 

 of the mercury in the short limb. A thread attached 

 to the float passes over a little wheel, and carries at 

 the other end a weight to counterpoise the float. 

 The rising and falling of the mercury column by 



