AXMcJhetving, to what degree Air is compreJftbU in Sea>vpatery 
at the depth of any number of feet from !♦ to 3 j. feet or ji /o- 
thom^ and theme for any number of *y: fathoms^ or 33. /^^^, to 
:^24i fathoms or l^^j feet^ 
SOme Members of the 7^oy«/ Sociefj did with two different 
forts of loftruments make divers Experiments for find- 
ing the Proportions of the Comprejfion of Air under Water/in 
the Month of July, at sheernefs, in the mouth of the River of 
Medrvay, at the time of high water, where the depth was then 
about 19 Fathom^ and the proportion of the weight of the 
Salt-watec to thatof the famequantity of FreCh water, taken 
out of the vivtrThames, was as 41 to 42. 
One of the Inftruments was a Glafs-bottle, that held a 
quart of water, having a bras ring faftned to the mouth of it, 
with a valve or flap, that open'd inward^ fo well fitted, that 
the bottle being filled more or lefs with water^none dropp'd 
out though forcibly ihaken* This^let down 33 foot into the 
water the mouth downwards^and after a litde ftay drawn up, 
was found to be fo very near half full of water, at feveral 
trials, that it was thought fit to flare the Compreffion of Air 
at that depth to that raeafure, wh'ch at other depths was 
found to hold the proportions fee down intheTable» 
The Quantity of the Compreffion was known by weighing 
theBottle with the water in it, after that a forcible depreffion 
of the Flap had made way for the eruption of the Comprefl 
Air (which kept it up even when the bottle was placed^wich 
the mouth upwards J and then filling the bottle full of the 
fame water, and weighing it again \ and lattly by weighing 
the bottle after the water was alllet forth ; the weight where- 
of being dediu^icd, the firft quantity of water weighed juft 
hjilf as much as the fecond, or fo near it that the fradlion was 
notconfiderable: Whence it was concluded, that the Quan- 
tity of the Air, that filled the bottle before it was immerfed in 
thewater, was,at the depth of 33 feer, compreft into half the 
fpace it took up beforehand fo proportionably at other depths. 
This was confirmed by repeated Experiments made with 
the other Inftrument^ which was a Cylinder of Glaf?, fome 
two foot long, clofc atone end, and having the other end 
drawn 
