( 217 ) 



One might construct a manometer entirely with tubes of that type, 

 each for r atmospheres, in which the additional pressure above n r 

 atmospheres might be read on a tube proof against n r atmospheres, 

 while each of the n preceding tubes were adjusted on a difference of 

 pressure of r atmospheres but this manometer when adjusted to a 

 high pressure, would contain more compressed gas than is necessary. 



This is not desirable, for with unaltered section of the tubes at 

 the place of the mercury-menisci, and with unaltered distance 

 of the latter, change of temperature in the gas confined between 

 the two columns of mercury, will bring about a displacement of the 

 menisci which will be the stronger as the volume of the confined 

 gas is larger. So we must try to limit as much as possible the volume 

 of the space filled with gas. This is also desirable in order not to 

 waste any gas during the operations; which gas, with a view to the 

 preservation of the apparatus and in order to enable us to apply the 

 corrections with exactitude must be perfectly dry and pure; finally 

 because the effect of a possible explosion of a tube filled with gas 

 at a high pressure, is in proportion of the (juantity of the gas 

 compressed therein. 



In order to economize gas we use the second type represented 

 by the tubes B^^B^ in figure 1, which is also easier to be handled 

 than .1. These tubes consist of a wider upper cylindrical vessel, 

 h and a lower cylindrical vessel, a, in which the mercury-menisci 

 are adjusted, of a capillary ^z, (comp. fig. 2) through which the mercury 

 rises and of the connecting capillaries /, and /t, bent downwards. 

 The diameter of both vessels is such as would be chosen for a 

 manometertube of class ^1, intended for the same pressure and 

 smaller in proportion to the augmentation of the pressure for which 

 it is destined. The following are the diameters of manometers for: 



Outer cliamctcr 



12.8 mM. 

 13.5 

 13.5 

 15.5 



In order to be able to take the lower cylinder as short as possible 

 we choose a narrow capillary for the ascension of the mercury, in 

 doing this however we must take into account that this has a 

 great influence on the motion of the mercury and makes that the 

 adjusting requires more time. As the co-efficient of friction of mer- 



15* 



