697 
of Edinburgh , Session 1879-80. 
As an inch of mercury exercises a pressure equal to 13 J inches of 
water, we have for the corrected depth 
D' = D {1 -0*03 (30 - H)} 
where D is the depth read off from the scale, and H is the barometric 
height. D' the corrected depth is given in fathoms. 
4. On the Compressibility of Glass. By J. Y. Buchanan. 
(Abstract,) 
The experiments related in this paper were undertaken with a 
view to determine, by actual observation, the effect produced on 
solids by hydraulic pressure. The instrument used consists of a 
hydraulic pump, which communicates with a steel receiver capable 
of holding instruments of considerable size, and also with a second 
receiver of peculiar form. This receiver consists essentially of a 
steel tube terminated at each end by thick glass tubes fitted tightly. 
It is tapped at the centre with two holes, the one to establish con- 
nection with the pump and the other to admit a pressure-gauge or 
manometer. The steel tube may be of any length, being limited 
only by the extent of laboratory accommodation at disposal. The 
tube which I am using at present has a length of a little over six 
feet and an internal diameter of about three-tenths of an inch. The 
solid to be experimented on must be in the form of rod or wire, and 
must, at the ends, at least, be sufficiently small to be able to enter 
the terminal glass tubes, which have a bore of 0*08 inch, and an 
external diameter of 0*42 inch. The length of the rod or wire is 
such that, when it rests in the steel tube, its ends are visible in the 
glass terminations. 
The experiment is conducted as follows : — A microscope with 
micrometic eyepiece is brought to bear on each end of the rod or 
wire. These microscopes stand on substantial platforms, altogether 
independent of the hydraulic apparatus. The pressure is now raised 
to the desired height, as indicated by the manometer, and the ends 
of the rod are observed and their position with reference to the micro- 
meter noted. The pressure is then carefully relieved, and a displace- 
ment of both ends is seen to take place and its amplitude noted. 
The sum of the displacements of the ends, regard being had to their 
