DETERMINATION OF THE SPECIFIC HEAT OF WATER. 435 
The experiments were repeated two years later, on February 22, 1894, when two 
different determinations gave 
0°002096 
0:002072 
Mean . . 0:002084 
It is seen that the changes in atmospheric pressure, which may amount to several 
centimetres, may produce a change in the indications of a delicate thermometer which 
cannot be neglected. 
From the effect of external pressure we may easily deduce that of internal 
pressure. If Pa is the diminution of volume due to an increase of pressure P applied 
externally, and P the increase in volume of the same pressure is applied internally, 
then the diminution in volume due to a simultaneous increase of pressure P inside 
and outside is P(«# — 8), and this must also be equal to P/x, where « is the 
compressibility of the walls of the thermometer bulb. Hence 
a — B= 1/k.* 
The reading apparently is diminished when the thermometer is raised from the 
horizontal to the vertical position, not only because the glass bulb expands, but also 
because the mercury is compressed. If x«’ is the resistance to compression of mercury, 
a pressure P will have the same effect as an increase of volume P/«’ of the reservoir. 
Hence, if 8 now denotes the complete effect of unit increase of pressure applied 
internally on the reading in degrees of the thermometer, we obtain 
1 1 
Sait shee ay aa 
K K 
1 ae. . oF) 076 c : 
—; —— is the relative compressibility of mercury and glass, which is known. 
K K 
AMAGAT, whose measurements doubtless are the most accurate at our disposal, finds 
for glass of medium hardness the relative compressibility to be 1°707 x 107°, and 
for lead glass (cristal) 1°512 x 10~® (‘Annales de Chimie et de Physique,’ vol. 22, 
1891). For the French thermometer glass the coefficient will not differ much from 
the larger of these values, 
The numbers given refer to the atmosphere as unit pressure. It is more convenient 
* This relation is quite general and independent of the shape of the bulb, which need not be of the 
same thickness throughout, provided it is of homogeneous material. The relation in Gutttaumn’s book 
is proved in an elaborate manner for spheres and cylinders, and is then said to hold ‘no doubt only 
approximately ” for thermometer bulbs. 
[Since the above was written, Mr. Curun has published a paper (‘ Phil. Mag.,’ October, 1894), in 
which a proof of the relation between the internal and external pressure coefficients practically identical 
with the above has been given.—June, 1895. | 
a ix 8 
