694 
Proceedings of the Royal Society 
It is interesting to note the nearness in every case of the value 
of the specific heat per unit volume to unity. I am continuing my 
experiments, and hope later to be able to state the results of a more 
extended series of observations. 
3. On a “Navigational” Sounding Machine. 
By J. Y. Buchanan. 
The sounding machine of which the annexed figure is a represen- 
tation, is intended for use at considerable depths while the ship is 
proceeding on her course. It is therefore necessarily capable of 
indicating the depths independently of the length of 
sounding line or wire used. It possesses the further 
great practical advantage, that when it has served its 
purpose once it is immediately available for another 
sounding. 
It indicates primarily the extent to which a given 
volume of air at atmospheric pressure has been con- 
densed by the column of water to which it has been 
subjected. As the law of the compression of air is 
known, the depth reached by the instrument is at once 
deduced. 
The instrument consists of a tube A B, which may 
either be of uniform diameter, or made up of lengths of 
different diameters ; in the figure the instrument is 
represented as made of two sizes of tube. This answers 
all practical requirements. The lower end B is contracted 
into a nozzle so as to receive a piece of india-rubber tube 
which can be plugged with a glass rod. At the other 
end A, a piece of tube drawn out to a moderately fine 
point is inserted, the point being bent slightly round, 
and fused hermetically into the end of the tube. The 
end A is thus closed by a sort of crooked funnel, which 
on the upper side is contracted to an orifice, at least as 
small as that of the end of the tube, projecting inwards. 
The object of this is to sift the water and allow no 
solid particles to enter which will not pass through the 
lower orifice of the funnel. The instrument is cali- 
brated either by weighing or by measuring the water which it can 
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