Sea-GAGE, an instrument invented by Dr Hales and Dr Desaguliers, for finding the depth of the sea; the description whereof is this. AB (Plate CXV. fig. 1. no 1.) is the gage-bottle, in which is cemented the gage-tube F in the brass cap at G. The upper end of tube F is hermetically sealed, and the open lower end f is immersed in mercury, marked C, on which swims a small thickness or surface of treacle. On the top of the bottle is screwed a tube of brass HG, pierced with several holes to admit the water into the bottle AB. The body K is a weight hanging by its shank L, in a socket N, with a notch on one side at m, in which is fixed the catch l of the spring S, and, passing through the hole L, in the shank of the weight K, prevents its falling out when once hung on. On the top, in the upper part of the brass tube at H, is fixed a large empty ball, or full-blown bladder, I, which must not be so large, but that the weight K may be able to sink the whole under water.
The instrument, thus constructed, is used in the following manner. The weight K being hung on, the gage is let fall into deep water, and sinks to the bottom: the socket N is somewhat longer than the shank
L; and therefore, after the weight K comes to the bottom, the gage will continue to descend, till the lower part of the socket strikes against the weight: this gives liberty to the catch to fly out of the hole L, and let go the weight K: when this is done, the ball or bladder I, instantly buoys up the gage to the top of the water. While the gage is under water, the water having free access to the treacle and mercury in the bottle, will by its pressure force it up into the tube F, and the height to which it has been forced by the greatest pressure, viz. that at the bottom, will be shewn by the mark in the tube which the treacle leaves behind it, and which is the only use of the treacle. This shews into what space the whole air in the tube F is compressed; and consequently the height or depth of the water which by its weight produced that compression, which is the thing required.
If the gage-tube F be of glass, a scale might be drawn on it with the point of a diamond, shewing, by inspection, what height the water stands above the bottom. But the length of 10 inches is not sufficient for fathoming depths at sea, since that, when all the air in such a length of tube is compressed into half an inch, the depth of water is no more than 634 feet, which is not half a quarter of a mile.
If, to remedy this, we make use of a tube 50 inches long, which for strength may be a musket-barrel; and suppose the air compressed into an hundredth part of half an inch; then by saying, as 1 : 99 :: 400 : 39600 inches, or 3300 feet; even this is but little more than half a mile, or 2640 feet. But since it is reasonable to suppose the cavities of the sea bear some proportion to the mountainous parts of the land, some of which are more than three miles above the earth's surface; therefore, to explore such great depths, the doctor contrived a new form for his sea-gage, or rather for the gage-tube in it, as follows. BCDF (ibid. no 2.) is a hollow metalline globe communicating on the top with a long tube AB, whose capacity is a ninth part of that globe. On the lower part at D, it has also a short tube DE, to stand in the mercury and treacle. The air contained in the compound gage-tube is compressed by the water as before; but the degree of compression, or height to which the treacle has been forced, cannot there be seen through the tube; therefore, to answer that end, a slender rod of metal or wood, with a knob on the top of the tube AB, will receive the mark of the treacle, and shew it when taken out.
If the tube AB be 50 inches long, and of such a bore that every inch in length should be a cubic inch of air, and the contents of the globe and tube together 500 cubic inches; then, when the air is compressed within an hundredth part of the whole, it is evident the treacle will not approach nearer than 5 inches of the top of the tube, which will agree to the depth of 3300 feet of water as above. Twice this depth will compress the air into half that space nearly, viz. 2 inches, which correspond to 6600, which is a mile and a quarter. Again, half that space, or 1 inch, will shew double the former depth, viz. 13200 feet, or 2 miles; which is probably very nearly the greatest depth of the sea.