436 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
for our purpose to take as unit pressure that due to a column of mercury 1 centim. 
long, and for unit change of volume that corresponding to an increase in temperature 
of one degree. The relative coefficient of thermal expansion of glass and mercury 
between 10° and 20° C. being 1°598 x 1074, the apparent compressibility of 
mercury reduced to the new units will be found by dividing the above number. by 
76 X 15598 x 10~*. The relation between B and « then becomes 
B=a-+0°'000141. 
The number used by the Bureau International des Poids et Mesures, founded on 
some older determinations, is 0°000154, and the uncertainty of this term might be 
raised as an objection to the use of a thermometer in the horizontal position. But 
the whole term being of little importance, it does not matter much which of the 
different possible values we adopt. Thus the difference between GUILLAUME’S 
internal pressure correction and ours would produce a difference in the measured 
temperature range of our Baudin thermometer of one part in 25,000, and in the 
Tonnelot standard it would be equal to only the sixth part of that value. For the 
sake of uniformity we have continued to use GUILLAUME’S number, and therefore 
take for the Baudin 12,772, 
a = 0°002084 
B = 0:002238, 
where « and 6 measure in degrees the changes in the apparent temperature due to 
a change of pressure of 1 centim. of mercury applied to the ontside or inside 
respectively. 
A small correction was necessary to allow for the fact that the mercury in that 
part of the stem which is not immersed in the calorimeter is not necessarily at the same 
temperature as the water. To ascertain this correction two small thermometers were 
used, one placed in the space between the calorimeter and the disc (D, fig. 5) which 
covered the water jacket, while the other was placed above this disc about half-way 
between it and the upper end of the thread. We found that in the last period when 
the calorimeter had risen about 2°, that part of thread below the dise D could be 
assumed to be at a temperature 1° above that of the air, while that part of the 
thread which was above the disc had not appreciably changed its temperature. The 
correction was always small, as the whole rise was only 2° and only a few degrees of 
the stem were exposed to the air. 
The thermometer was always read by means cf a cathetometer telescope, the 
cathetometer stand being very convenient to move the telescope parallel to itself. 
The avoidance of parallax is of importance and can be easily secured in this way, 
provided the thread is always read when in the middle of the field. 
