The Locomotive . us: Let there be two barometrictubes, placed side by side so that the mercury may have the same temperature ineach, and each dipping at the bottom, into the same vessel of mercury. If thevacua in the upper parts of these tubes are both perfect, then the two columns ofmercury will stand at precisely the same height. If, now, a small quantity ofwater be introduced into one of the vacuous spaces, it will partially evaporate,the evaporation ceasing when the space containing the water becomes filled withvapor of the particular density corresponding to the temperature to which theba
The Locomotive . us: Let there be two barometrictubes, placed side by side so that the mercury may have the same temperature ineach, and each dipping at the bottom, into the same vessel of mercury. If thevacua in the upper parts of these tubes are both perfect, then the two columns ofmercury will stand at precisely the same height. If, now, a small quantity ofwater be introduced into one of the vacuous spaces, it will partially evaporate,the evaporation ceasing when the space containing the water becomes filled withvapor of the particular density corresponding to the temperature to which thebarometric column is exposed. The mercury column in the tube containing thewater will meanwhile fall by an amount which, roughly speaking, corresponds to 88 THE LOCOMOTIVE [July, the pressure of the water vapor, at the given temperature. In practice, certaincorrections have to be applied to the observed difference in the barometric read-ing. For example, in order that we may be sure that the vapor present is really. Fig. i. — Regnaults Apparatus for Low Pressures. 1906.] THE LOCOMOTIVE. 89 saturated, it is necessary to introduce a slight excess of water, over and abovethat which would be sufficient to just saturate the space in the barometer; andif this excess remains in contact with the top of the mercury column, we shallhave to make allowance for the amount by which the column is depressed by theweight of the unevaporated water. The surface tension of the mercury, whichcauses the top of the column to assume the familiar rounded shape, is also likelyto be modified by the presence of the water, and the water itself has a surfacetension which acts in the opposite direction to that of the mercury, and allow-ance has to be made for this fact. Regnault investigated all these sources of er-ror, and either satisfied himself that the> are of no practical importance, or elsedetermined the magnitude of the errors that they introduce, and made proper al-lowances. In some of his experi
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