MECHANICAL STRETCHING OF LIQUIDS. 
359 
The Bourdon gauge used was employed for other observations to be described later, 
and when all were completed it was re-tested for me by Mr. C. F. Casella, by means 
of a mercury column, at the beginning of December, 1891, up to 14 atmospheres, with 
the following results :— 
Bourdon gauge. 
Mercurial column. 
atmospheres. 
0 
pounds per sq. inch. 
0 
pounds per sq. inch. 
0 
2 
30 
30 
4 
60 
62 
6 
90 
89 
8 
120 
121-5 
10 
150 
151-5 
12 
180 
179 
14 
210 
209 
The deviations from the original calibration up to this range will be observed 
almost to fall within the errors of reading, and by an independent comparison, which 
I made myself in the same month, of this gauge with two official standard gauges, by 
E. Bourdon, belonging to the Admiralty, I could not detect with any certainty any 
appreciable divergence of the readings, which I think may therefore be taken as 
correct within 1 per cent. 
It may be mentioned, that in May, 1889, pressures of 36 gauge-atmospheres had 
been applied to the tonometer, giving a mean rise of 3'56 millims. per atmosphere, 
but in December, 1891, the higher readings of the gauge were found to be as much 
as 8 per cent, too low. The application of this correction reduced the reading to 3'275 
millims. which accords well with the results already quoted, but on account of the 
doubt as to whether the gauge was ever correct at this part of the scale, it seemed 
better to exclude these observations altogether in determining the tonometer calibra¬ 
tion, and to rely only on those within the range that I had myself tested. 
It was afterwards observed by using the tonometer as a thermometer that the rise 
of the mercury due to a rise of temperature of 1 0, 85 C. was 2 centims., showing that 
the deformation of the bulb due to the maximum tension afterwards obtained, viz. 
17 atmospheres, amounted to only about 1/1000th of its whole volume. 
Measurement of the Strain or Extension. 
In order to ascertain the extension at any instant, the liquid was caused to let go 
its hold, and thus spring back to its unstretched volume, and then the volume of the 
space left empty of all but vapour was measured. 
For this purpose the straight tube AB was traversed at F by the fine platinum 
ware, already mentioned, running across the centre of the tube. One of the 
projecting ends of this wire was connected by a stouter copper wire to one pole of an 
