THE MECHANICAL EQUIVALENT OF HEAT. 
427 
[ The calibration of those portions of the thermometer between our “ fixed points ” 
was conducted in the same maimer as the calibration over the whole range. 
[Note by E, H. Geiffiths, added March 31st, 1893. 
On February 14 I received the thermometer by M. Tonnelot (No. 11048), 
which had been standardized both by Dr. Guillaume, and under his direction, at the 
Bureau International des Folds et Mesures. 
This thermometer is of hard glass, with transparent stem, and is divided into 
tenths of a degree Centigrade. The scale is continuous from — 3° to 27° C. when there 
occurs a bulb, then a narrow portion giving readings 48° to 52° C., then a second bulb, 
and, finally, readings from 98°‘5 to 101°'5 C. The length of each degree is about 
10 mlllims. 
The study of this thermometer has been a most exhaustive one; two separate 
calibrations were performed for the most useful part (10° to 26°’5 C .)—the greatest 
difference between any two values given by the separate determinations being 
0°'0028 C. The fundamental interval and the coefficients of external and internal 
pressure were repeatedly determined. Several hours must have been occupied in the 
mere copying of the observations forwarded to me—although the record consists, 
almost entirely, of numerals. I mention this as indicating the labour which has been 
devoted to this standardization, and I owe my warmest thanks to the Bureau Inter¬ 
national for the care and attention bestowed upon it. 
The papers include printed tables for the reduction of the corrected mercury readings 
both to the hydrogen and the nitrogen scale as resulting from the comparisons of Dr. 
Chappuis. 
The comparison between this thermometer (denoted hereafter by P) and the 
thermometer was conducted in the outer tank, for had the comparison been made 
in the .calorimeter, the lower readings of P would have been invisible. E„ was, 
therefore, under the same conditions as during our J experiments, except in the 
following particulars :— 
(l.) The external pressure was increased by about 110 millims. of water. A series 
of observations proved that the resulting correction was — '3 millim. (rather less 
than 0°'001 C.) and this correction was always made before entering the reading 
, ofE;„. 
(2.) When in the calorimeter, the lower portion of the stem passed through tubes 
washed by the tank water. About 60 millims. were thus maintained at a constant 
temperature, whatever the extent of alteration in the bulb temperature. About 
50 minims, intervened between the top of the calorimeter and the lower end of this 
constant temperature portion. We may consider that this 50 millims. assumed the 
mean temperature between the tank and the calorimeter. Thus a length of (60 + 25) 
millims. of the stem, which during our previous experiments was at the tank tem¬ 
perature 19°'26, assumed, during this comparison, the temperature of the bulb 6-^. 
3 I 2 
