434 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
thermometer, and that our calibration in 1892 was satisfactory. The numbers 
obtained from the single thread by Gay-Lussac’s method are practically identical 
with those obtained by the more complete method. 
The temperatures for which the corrections have been obtained being too far apart 
to allow us to interpolate safely for the intermediate points, a thread of 2°°5 was 
broken off, and by means of it the corrections for every half-degree could be deduced. 
The divisions of the thermometer were not equidistant, but the maker had attempted 
to correct for inequality of bore. This method, much in use at one time, causes a 
good deal of unnecessary labour, as already pointed out by GuiLLAUME, whenever the 
instrument is to be used for accurate work; its subsequent calibration cannot be 
dispensed with, and the lengths of the divisions must be measured one by one, as 
there is no clue where a sudden change in the length may occur. We give in an 
appendix the method adopted to deduce the final corrections for each division of the 
thermometer. 
Pressure Corrections. 
The reservoir of a thermometer is generally subjected to an internal pressure due 
to the column of mercury forming the thread, and this pressure causes quite an 
appreciable increase in volume. If the thermometer is always used in the vertical 
position, no error is caused in assuming uniformity of bore; the effect of internal 
pressure will only be a shortening of the distance between the fundamental points. 
Even in this case, however, care must be taken that the range for which the 
thermometer is used does not include bulbs like those which are placed above the 
freezing point in calorimetric thermometers. 
A comparison between two thermometers is more easily conducted in a horizontal 
than in a vertical position ; and in that case, of course, the effect of pressure must be 
taken account of. 
The effects of external pressure are easily measured by suspending the thermometer 
in a vessel in which the pressure can be quickly changed from the atmospheric 
pressure to one of a few centimetres only. 
We have used a method which is a slight modification of that given in GUILLAUME’S 
book. It is unnecessary to enter into long details, as the method will be described in 
another communication. Three experiments were made on March 21, 1892. The 
change of pressure was a sudden diminution from the atmospheric pressure to one of 
27 centims. The observations, when reduced, give 
0°:002088 
0°:002085 
0°-002080 
Mean . . 0°:002084 
as the effect due to 1 centim. of mercury at 0° C. at the latitude of Manchester. 
