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MESSliS. T. E. THORPE AND J. W. RODG-ER ON THE RELATIONS 
correction, which also included a small scale error, to the distance between the 
scales. The variation in length of the manometer between the mean and extreme 
temperatures of the laboratory may be neglected. The true length of the water 
column on a scale correct at 4°, having the temperature of the atmosphere, could 
thus he got with ease. To express this as a head of water at the temperature of 4°, 
advantage was taken of the fact that when the same two scales were used the pres¬ 
sure head was almost the same, and thus the correction of the head for change in 
density of the water depended only on the temjDerature. A table was therefore con¬ 
structed from which by inspection the correction to be applied to the head at any 
temperature in order to give the height of a column of unit density could be obtained. 
In order to find the mean effective pressure, two corrections have to be applied to 
this head of water. 
1. Correction for Inequalities in the Atmospheric Pressure on the Liquid Surfaces 
in the Manometer and CUschrometer. —A head of air, assumed to have the mean 
atmospheric temperature and pressure, and having a height equal to the difference 
between the upper level of liquid in the manometer and the mean level of liquid in 
the glischrometer, opposes the flow of liquid through the capillary. Acting in the 
same direction is a head of air having the atmospheric temperature and a pressure 
equal to that of the atmosphere plus that indicated by the manometer, and having a 
height equal to the difference in level of the lower liquid surface of the manometer 
and the mean liquid level in the glischrometer. The mean value of this correction 
for the outer scales was O'17 centim. of water at 4^ ; for the inner scales the correction 
was O'] 3 centim. 
These values are not appreciably affected by changes in atmospheric density, and 
were therefore applied once and for all as corrections to the distance between the scales. 
2. Correction for Change of Head of Liquid in the Glischrometer .—This correction, 
which is needed to eliminate the effect of the alteration in the heads of liquid in 
the two limbs, is by far the more difficult to ascertain. 
If the limbs of the glischrometer had been identical in all respects, it would have 
been possible to arrange the working volume of liquid before each flow, so that the 
pressure produced by the head of liquid acting in unison with the pressure of the 
air-reservoir during the first half of the time of flow, would have been cancelled by 
an equal back pressure during the second half. As it was impossible to obtain the 
limbs exactly similar, and as on this account the mean head of liquid accelerating the 
flow differed from that retarding it, it became necessary to estimate the exact value 
of the effective pressure due to this cause, and provide some means of ascertaining 
its effect at any tem])erature with any liquid. This was rendered possible by the 
use in each flow of a constant working volume of liquid, as already described. To 
obtain the effective pressure called into play during the flow from, say, the right 
limb, it was necessary to take account of the fact that the rate of change of pressure 
was largely influenced by the varying diameter of the limb. A paper millimeter 
