542 
Proceedings of the Royal Society 
press gently on its lower side, without entering more than infinitesi- 
mally into the space between it and the surrounding glass tube 
(the condition of the upper hounding surface of the mercury in this 
respect being easily seen by the observer looking at it through the 
glass tube). When the piston is being lowered, the stopcock in the 
branch leading from the upper cistern is closed, and the one in 
the branch leading to the lower cistern is opened enough to let the 
mercury go down before the piston, instead of being forced to any 
sensible distance into the space between it and the surrounding tube, 
but not enough to allow it to part company with the lower surface 
of the piston. The manometer is simply a mercury barometer of the 
form commonly called a siphon barometer, with its lower end not 
open to the air but connected to the lower end of the manometric 
capillary. This connection is made below the level of the mercury 
in the following manner. The lower end of the capillary widens 
into a small glass bell or stout tube of glass of about 2 centimetres 
bore and 2 centimetres depth, with its lip ground flat like the 
receiver of an air-pump. The lip or upper edge of the open cistern 
of the barometer (that is to say, the cistern which would be open 
to the atmosphere were it used as an ordinary barometer) is also 
ground flat, and the two lips are pressed together with a greased 
leather washer between them to obviate risk of breaking the 
glass, and to facilitate the making of the joint mercury tight. 
To keep this joint perennially good, and to make quite sure that 
no air shall ever leak in, in case of the interior pressure 
being at any time less than the external barometric pressure or 
being arranged to be so always, it is preserved and caulked 
by an external mercury jacket not shown in the drawing. 
The mercury in the thus constituted lower reservoir of the 
manometer is above the level of the leather joint, and the space in 
the upper part of the reservoir over the surface of the mercury, 
up to a little distance into the capillary above, is occupied by 
a fixed oil or some other practically vapourless liquid. This 
oil or other liquid is introduced for the purpose of guarding 
against error in the reckoning of the whole bulk of the ther- 
mometric gas, on account of slight irregular changes in the 
capillary depression of the border of the mercury surface in the 
reservoir. 
