of sea waters, in different parts of the ocean, &c. 167 
I shall now proceed to state the results which I have ob- 
tained in regard to the specific gravities of sea water ; and for 
the sake of conciseness I shall present them in the form of 
again by means of a small spring or wedge. It remained open so long as it descended ; 
but when pulled up again, the wedge was thrown out of its place, and this obstacle 
being removed, the box closed itself instantly. This was effected by means of a small 
fly-wheel, so confined as to admit of no motion during the descent of the machine; but 
the moment it began to ascend, the pressure of the water communicated to the wheel 
a rotatory mot on, which, through a little mechanical contrivance, disengaged the 
spring and clcsed the box. 
Another, and more elaborate invention, of the same kind, (a description of which may 
be found in the c Journal of Science and the Arts, V ol. V.) was devised last year by Sir 
Humphry Davy, and was repeatedly used in the late voyages towards the North Pole ; 
andin fact, many of the specimens of waters from those regions, which I have examined, 
were raised by means of that ingenious, though I apprehend, not unexceptionable ap- 
paratus. 
The principle of this instrument may be stated in a few words. It consists in a 
strong copper bottle of an oblong shape, closed at its neck by a stop-cock. To this 
bottle is attached laterally, and in a parallel direction, a metallic tube closed at the 
top and open at the bottom, with an air-tight piston moving within the tube. As the 
open end of the tube therefore descends into the sea along with the bottle, the piston 
which closes the orifice of the tube is gradually forced upwards into it, as the ma- 
chine sinks, the air within it being proportionally compressed ; but when the piston 
has reached a certain part of the tube, it meets with a catch and opens the cock of the 
bottle, which, of course, instantly fills with water ; and there is an ingenious contriv- 
ance by which the machine may be set before hand, so as not to let in the water till a 
certain known degree of pressure is made by the superincumbent column. 
Captain Ross, in his account of his ‘ Voyage to the Arctic regions’ (Appendix, 
page cxxiv.), makes the following observation : e Sir Humphry Davy’s apparatus 
answered the purpose for which it was intended ; but it did not close so as to prevent 
the water from escaping or mixing with that nearer the surface as it came up.’ This 
objection however, which might probably be removed, is of no great importance, as 
the aperture in question being very small, and situated at the upper end of the vessel, 
there is no risk of any sensible change taking place in the contents of the vessel through 
that aperture, while traversing the upper strata. But the machine appears liabl to the 
