COMPRESSIBILITY OF GLASS. 593 
excess being sometimes at the one end and sometimes at the other. The 
effect of the rise of pressure is to extend the containing tube, and to compress 
_ the contained rod. On the relief of pressure the tube shortens again, and the 
rod recovers its length, and there is necessarily a sliding of the one on the 
other, and it depends entirely on minute local circumstances whether the rod 
finds it easier to return to its original relative position or to another. In some 
experiments made previously to the date of those quoted in Table II., the rod 
had greater freedom of motion longitudinally, and it happened several times 
that it crept bodily to the one end, necessitating the opening of the apparatus to 
replace it in a position suited to observation. Afterwards stops were placed in 
the tube, which, while setting limits to the crawling motion, did not in any way 
interfere with the expansion and contraction. The results of these previous 
experiments are not included in the Table, because they were merely tentative 
in order to learn the details of the kind of experimentation ; and further, be- 
cause in the microscope at the east end the power used was very low, and the 
micrometer insufficiently delicate. 
In the left hand columns the individual experimental data are given. The 
arithmetrical means of the manometric pressures and of the total micrometric 
expansions are taken for-each series. These mean results are then further 
developed on the right hand side of the table. First the temperature is given, 
T. This remains always very constant, as it was the temperature of the room, 
which varied very little. It was further controlled between each experiment 
by the reading of the manometer when the pressure was reduced to that of the 
atmosphere. The pressure in atmospheres (P) is obtained, as explained above, 
by multiplying the manometric pressure (A) by 3°13, 
PSs 3x A, 
The linear compression (F) for pressure (P) is given by multiplying the micro- 
metric expansion by the value of a division, or 0:000417”, 
F=0-000417 x D, 
108 
H=rzo5F. 
H is the linear compression in inches of a rod one million inches long for 
pressure P. 
ET 
K=5 
is the same per atmosphere or the linear compressibility of the glass. 
N=3K 
is the cubical compressibility of the same glass. 
These results are summarised in Table III. 
