696 
Proceedings of the Royal Society 
the reading noted. The calculated reading which it ought to have 
shown on the above assumption is put down, and it will be seen 
how closely the two agree. 
This instrument is especially adapted for surveying- work where 
lines of sounding in comparatively deep water have to be run for 
considerable distances, as in the English or Irish Channels, or the 
North Sea. The sounding line or wire is arranged to pay over the 
stern, and arrangements are made for promptly heaving it in on 
bottom being reached. The engines are kept going at a convenient 
and uniform rate, and the revolution counteracted; it is then easy 
with good organisation to take soundings every hundred, two hundred, 
five hundred, or thousand revolutions. In the want of a revolution- 
counter, which, however, ought to be fitted to every marine engine, 
equally good results can be obtained if the engineer pays attention 
to keep his engine going uniformly, checking its rate at frequent 
intervals by counting the revolutions in a minute, by making the 
soundings at regular intervals of time. The number of revolutions 
made by the engines in a vessel of known capabilities is a very 
accurate means of ascertaining the distance run, and if this method 
were adopted the deep sounding of a survey could be worked off 
with great expedition and accuracy. 
There is one property of this sounding -machine which must not 
be lost sight of, namely, that it registers the sum total of the increments 
of pressure. If it is to give the depth correctly it must both descend 
and ascend without interruption, and in the work for which it is 
designed this condition is always fulfilled. Suppose, for instance, it 
be sunk to 20 fathoms and be then drawn up to 10 fathoms, the 
corresponding quantity of air will be eliminated. If it is now again 
sunk to 20 fathoms, the place of the air which left while it was 
rising from 20 to 10 fathoms, will be taken by water, and if it now 
be brought to the surface it will of course register a depth greater 
than 20 fathoms. 
It follows from the principle of this instrument that the value of 
its graduation scale will vary to a certain extent with the barometric 
pressure. Eor purposes of navigation the error so caused is neglige- 
able, but if used for surveying purposes a correction must be 
applied. 
The instrument is graduated for a barometric pressure of 30 inches. 
