of Edinburgh, Session 1879-80. 
695 
hold. As the volume assumed by the air is inversely as the pressure, 
it can at once be graduated into fathoms or other units of depth. 
For use it is enclosed in a brass tube, and attached to the sound- 
ing line and allowed to sink. As it sinks the air is compressed, and 
its place taken by water which enters through the funnel, and being 
delivered in a fine stream against the walls of the tube runs down 
and collects at the lower end. On bringing the instrument to the 
surface again the water cannot get out, but the compressed air 
occupying the upper portion of the tube gradually expands through 
the orifice, which it thus completely occupies and prevents the 
entrance of water. Arrived at the surface, the depth is read off 
directly if the instrument is divided into units of depth, or if its 
scale is arbitrary the depth is found from a table constructed accord- 
ing to the results of calibration. When this has been done the plug 
C is removed, the water runs out, the plug is replaced, and the 
instrument is ready for use. 
Last summer I had frequent occasion to test the accuracy of one 
of those instruments in the deep waters of Loch Fyne, and found it 
most satisfactory. The instrument which I used had an arbitrary 
scale of equal lengths, and had been very carefully calibrated by 
weight. From the results of the calibration I constructed a table 
of depths corresponding to the graduation, on the assumption that it 
was inversely proportional to the volume assumed by the air, and 
neglecting any effect of this pressure in causing increased absorption. 
The following results obtained on 13th June 1879, while anchored 
in 87 fathoms off Garrock Head in Frith of Clyde, will show the 
Depth (fathoms). 
20 
40 
80 
, 
90-5 
182-0 
243-0 
Found < 
90-0 
183-0 
245-0 
( Mean, 
90-25 
182-5 
244-0 
Calculated, .... 
89-0 
182-6 
243-0 
Difference, .... 
0-25 
— 0-1 
1-0 
close agreement between the observed and the calculated depth. 
The instrument was sent down twice to 20, 40, and 80 fathoms, and 
4 o 
VOL. x. 
