164 Dr. Marcet on the specific gravity, and temperature 
temperature.* The balance I used was one which was sen- 
sibly affected by T %th part of a grain ; but I did not think it 
necessary to use smaller weights than ^th P art °f a g ra i n : 
so that whenever, in the annexed tables, smaller weights are 
expressed, in the sixth decimal figure, such very minute parts 
must not be understood to have been obtained from actual ex- 
periment, since they resulted, by calculation, from the conver- 
sion of the weights actually obtained into the usual standard 
of 1000 parts. 
The first idea of the apparatus which I contrived for raising 
water from the bottom of the sea, occurred to me about ten 
years ago, on accidentally seeing in an instrument maker's 
shop, a machine, said to be the identical one which was used 
for a similar purpose by Dr. Irving, in Captain Phipps’s 
(since Lord Mulgrave) expedition. -f This consisted simply 
in a cylindrical vessel having an opening at the top, and a si- 
milar one at the bottom, each closed by a flap or valve opening 
only upwards, and moving freely upon hinges. When this 
sea water was less salt between the tropics than between the coasts of Spain and Tene- 
riffe ; and his observations 6eem to lead to the inference, that there is a specific gravity 
peculiar to the water of each zone; a conjecture, however, which the facts collected in 
this paper do not appear to confirm. 
* It may perhaps be worth while to mention a small improvement which was intro- 
duced in the vessel used for weighing the waters. The apparatus consisted in a thin 
phial, nearly spherical, containing between five and six hundred grains of distilled 
water, and having a very light ground glass stopper. But as I had observed on former 
occasions that such phials were apt to burst on the stopper being forced in, from the 
compression of the liquid with which they were filled, I had the stopper made with a 
very small longitudinal aperture through it, so as to allow a minute quantity of water 
to ooze out; and this was very easily accomplished (by Mr. Newman, of Lisle-street) 
bv forming the stopper of a portion of thermometer tube, the bore of which perfectly 
answered the desired purpose. 
•f Ph 1 pps’s Voyage to the North Pole in 1773. 
