( 30 ) 
It would be very difficult to keep the temperatures at which those 
isothermals are to be determined so constant that during the time 
required for a determination with the open standard manometer no 
variations can occur which would influence the pressure to be 
measured more than the errors of adjustment. The best thing 
to be done seemed to measure the pressure in the piezometers of 
constant volume filled with gas at a low temperature (a following 
communication will treat of its construction and use) by means of 
gas-gauges which have themselves been compared with the open 
manometer of reduced height, and to construct these closed mano- 
meters so that they are not much inferior either in sensibility or 
reliability to the standard open manometer. 
Ip order to render the indications of the piezometers or mano- 
meters as reliable as possible the glass tubes in which the gas is 
compressed have been taken so wide as appeared to be compatible 
with a sufficient power of resistance and would not render too diffi- 
cult the handling. The chance that a perceptible quantity of the 
gas, albeit in extremely small bubbles, should adhere to the walls 
when the mercury rises, as well as the influence of a deviating 
behaviour of the gas-layers at the surface of the glass, becomes less 
as the tube is wider. Moreover the more regular shape of the me- 
niscus renders the determination of the volume more accurate and 
diminishes the influence of capillary disturbances on the measure- 
ment of the pressure. 
The manometer-tubes may be cleaned and refilled without in- 
validating the determination of constants once made. This is of great 
importance, also because it enables us to apply a differential method 
when comparing the isothermals of two gases or mixtures of gases. 
For if we dispose of two manometers of the kind to be described, 
we can compress these gases simultaneously under the same pressure, 
and interchange them in the two sets; so the errors which the 
apparatus might still show, are eliminated for the greater part. 
For the rest the closed manometers are so constructed that the 
normal volume can be very accurately determined not only at the 
beginning of the measurements made under pressure, but at any time 
we should want to. In order to do so the manometer-tube filled 
with gas may be taken from the apparatus, placed in a space of 
constant temperature, — where the difference between the pressure of 
the enclosed gas and the atmosphere can be measured, taking into 
account the capillary depression —, and may be replaced in the appa- 
ratus being still as clean as before while the quantity of enclosed 
gas does not undergo the least change during this operation. The 
