i66 Dr. Marcet on the specific gravity, and temperature 
and pervious till the weight touches the bottom, at which mo- 
ment it closes itself, and remains, in that state. The water 
brought up in this machine, therefore, can only come from the 
bottom, and from no intermediate depth ; for whenever the 
apparatus fails in reaching the bottom, it continues pervious, 
and on being raised out of the water is found open, and emp- 
tied of its contents. The advantage however of raising water 
from the bottom with certainty, whenever the bottom can be 
reached, is a valuable one, and has already enabled me to 
ascertain some curious points respecting the sea of Marmora, as 
will be presently seen ; and if some mode could be contrived 
of closing the machine at any desired intermediate distance 
from the surface (an object to which I have directed the at- 
tention of the ingenious instrument-maker, Mr. Newman), it 
it would then answer every desired purpose.* 
* Since this paper was written, Mr. Newm a n has succeeded in constructing a 
machine, (just in time to he put on board of Lieut. Parry’s ship, about to sail to 
Baffin’s Bay), which seems well calculated to answer the desired object, under any 
circumstances. The principle is essentially the same as that of the machine which 
I have just described, namely, that of closing itself when it touches the bottom; but 
with this material addition, that the valves, when the bottom cannot be reached, may 
be closed at any given depth, by causing a weight to descend along the cord to which 
the machine is suspended, till it comes into contact with it, and closes the valves by an 
appropriate contrivance. A sketch of this machine, with its explanation, is also an- 
nexed at the end of this paper, Plate XII. 
Among the attempts which have been made to bring up water from any given 
depth, I should not omit to mention an ingenious contrivance of Mr. Tennant, 
which he thought calculated to answer the purpose tolerably well, and which, as will 
be seen hereafter, was used some years ago with apparent success. 
Mr. Tennant’s contrivance consisted in a wooden box, capable of holding a few 
ounces of water, and opening like a snuff-box, by a lid moving upon hinges, and fit- 
ting water-tight. The box, when closed, was forcibly kept in that state by a spring ; but 
when about to be used, it was partially opened, and was prevented from shutting itself 
