432 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
A thermometer plunged into a calorimeter can only be read with convenience when 
the thread is a few inches above the level of the water; and if the length of a degree 
is short, the correction for the emergent stem becomes serious. It is important that 
this correction should be small, as it cannot be ascertained with great accuracy. © 
Hence it is better for calorimetric measurements to use a thermometer with an open 
seale so that nearly the whole thread may be immersed. The open scale has the 
further advantage of being more easily read. It is generally supposed that the 
accuracy of thermometer measurements is increased by using a thermometer in which 
the length of a degree is made very large, and the manufacture of such thermometers 
has been pushed to ridiculous extremes. Our experience confirms the conclusion drawn 
by GuitLAumE (“ Thermométrie,” p. 180), that the uncertainties in the indications 
of a thermometer measured in degrees increase with an increase in the length of the 
degree, and that, therefore, apart from the convenience of reading, the advantage 
altogether is in favour of a short degree. When the whole thermometer can be 
plunged in water and the temperature of the water is kept sufficiently steady to 
allow a careful reading, a thermometer such as that of TonnELor, divided into tenths 
of a degree, the divisions being half a millimetre apart, allows as accurate a deter- 
mination of temperature as any mercury thermometer can give us at present. 
It was the uncertainty of the correction of the emergent stem which finally 
decided us to use in our calorimeter a thermometer with an open scale ; but it was 
essential that it should be made of the same glass as the standard. We chose one 
by Baupin (No. 12772), having a length of degree of 3°1 centims. approximately, 
and being divided into fiftieths. It contained according to the statement of the 
maker 33°0 grams. of mercury. It could be read at the freezing-point and between 
12° and 23°. The stem was shortened by an expansion in the glass above the zero 
point insuch a way that the distance from the division which marked 12° to the 
centre of the main bulb was only 8°1 centims. The radius of the bore can be 
calculated from the weight of mercury, and is thus found to be 0°0061 centim. 
From experiments made under varying pressures, the thickness of the glass bulb can 
be roughly estimated as ‘066 centim. In order to allow us to use the thermometer 
for our purpose it was necessary (1) to calibrate the thermometer, (2) to compare it 
with the standard, (3) to determine carefully the pressure coetticient, as the comparison 
had to be conducted with the thermometer in a horizontal position. 
The Calibration. 
The calibration was conducted according to the method of THrEssEN, as described by 
GuimtaumE. Threads were broken off, having a length of very nearly one, two, &c., up 
to ten degrees, and these threads were pushed forward from degree to degree. Their 
lengths were measured on the thermometer in the usual way, an estimate of the over- 
lapping of the mercury to one-hundredth part of a division being made by the eye, 
