Sea GAGE, an instrument invented by Dr Hales and Dr Desaguliers for finding the depth of the sea; the description whereof is this. AB (fig. 1.) is the gage bottle, in which is cemented the gage tube Ff in the brass cape 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 screw-
ed 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 / 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 underwater.
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 Ff, and the height to which it has been forced by the greatest pressure, viz. that at the bottom, will be shown by the mark in the tube which the treacle leaves behind it, and which is the only use of the treacle. This shows into what space the whole air in the tube Ff 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 Ff be of glass, a scale might be drawn on it with the point of a diamond, showing, 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 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 (fig. 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 show it when taken out.
If the tube AB be 50 inches long, and of such a bore:
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 five 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. 2½ inches, which correspond to 6600, which is a mile and a quarter. Again, half that space, or 1¼ inch, will show double the former depth, viz. 13200 feet, or 2½ miles; which is probably very nearly the greatest depth of the sea.
Bucket Sea GAGE, an instrument contrived by Dr Hales to find the different degrees of coolness and saltiness of the sea, at different depths: it consists of a common household pail or bucket, with two heads: These heads have each a round hole in the middle, about four inches in diameter, covered with square valves opening upward; and that they may both open and shut together, there is a small iron rod fixed to the upper part of the lower valve, and the other end to the lower side of the upper valve. So that as the bucket descends with its sinking weight into the sea, both the valves may open by the force of the water, which by that means has a free passage through the bucket. But when the bucket is drawn up, then both the valves shut by the force of the water at the upper part of the bucket; so that the bucket is drawn up full of the lowest sea water to which it has descended. When the bucket is drawn up, the mercurial thermometer fixed in it is examined; but great care must be taken to observe the degree at which the mercury stands, before the lower part of the thermometer is taken out of the water in the bucket, lest it be affected by the different temperature of the air. In order to keep the bucket in a right position, there are four cords fixed to it, reaching about three feet below it; to which the sinking weight is fixed. The result of several trials with this gage was, that when it was let down to different depths, from 360 feet to 5346 feet, in lat. 25. 13. N. and long. 25. 12. W. it was discovered by the thermometer, that the cold increased gradually in proportion to the depths, till it descended to 3930 feet, viz. near ¾ of a mile, whence the mercury in the thermometer came up at 53°; and though it was afterwards sunk to 5346 feet, i. e. a mile and 66 feet, it came up no lower: the warmth of the water upon the surface, and that of the air, was all that time 84°. When the water in the bucket was become of the same temperature with that on the surface of the sea, equal quantities of both were weighed and tried by the hydrometer; that from below was found to be the heaviest, and consequently the saltiest.
Dr Hales was probably led to the construction of this sea gage from an instrument invented by Dr Hook, and designed for the same purpose. This consists of a square wooden bucket C, whose bottoms are so contrived, that as the weight of A sinks the iron B, to which the bucket C is fastened by two handles D, D, on the end of which are the moveable bottoms or valves EE, and thereby draws down the bucket, the resistance of the water keeps up the bucket in the posture C, whereby the water, whilst the bucket was descending, hath a free passage through it; whereas, as soon as the
bucket is pulled upwards by the line F, the resistance of the water to that motion beats the bucket downwards, and keeps it in the posture G, whereby the included water is kept from getting out, and the ambient water kept from getting in. Phil. Trans. N° ix. p. 149. and N° xxiv. p. 447. or Abr. vol. ii. p. 260.
Aqueo-mercurial GAGE, is the name of an apparatus contrived by Dr Hales, and applied in various forms to the branches of trees, in order to determine the force with which they imbibe moisture. Let e r, Fig. 4. be a cylindric glass, e. gr. of an inch diameter within, and eight inches long. Into this glass is introduced the branch of a young thriving apple tree b, about three feet long, with lateral branches; the diameter of the transverse cut i being ¼ths of an inch. Having fitted the joint r to the tube at r, by folding a piece of sheep's skin round the stem, it is cemented with a mixture of bees wax and turpentine melted together, in such proportion as to make a very stiff clammy paste when cold, and over the cement folds of wet bladders are bound firmly with pack thread. To the lower end e of the large tube, a smaller tube x e is cemented, being about ¼ of an inch diameter, and 18 inches long, and in substance full ¼ of an inch thick. These tubes are cemented together at e with common hard brick dust or powdered chalk cemented, and the joint is farther secured with the cement of bees wax and turpentine, over which a wet bladder is bound. The apparatus being thus prepared, the branch is turned downwards, and the glass tube upwards, and then both tubes are filled with water; with the finger applied to the open end of the small tube, it is inverted and immersed in the glass cistern x, full of mercury and water. In this situation the lower end of the branch was immersed six inches in water, viz. from r to i; the water was imbibed by the branch at its transverse cut i; and during its ascent into the sap vessels of the branch, the mercury rose in the tube x e from the cistern x, so that in half an hour it was risen 5¾ inches high, as far as x. The height of the mercury indicated, in some measure, the force with which the sap was imbibed, though not the whole force; because, while the water was imbibed by the branch, its transverse cut was covered with innumerable little hemispheres of air, and many air bubbles issued out of the sap vessels, which partly filled the tube x e, as the water was drawn out of it: and therefore the height of the mercury could only be proportionable to the excess of the quantity of water drawn off above the quantity of the air which issued out of the wood. If the quantity of air issuing from the wood had been equal to the quantity of water imbibed, it is plain that the mercury could not rise at all, because there would be no room for it in the tube: but if nine parts in twelve of the water be imbibed by the branch, and only three such parts of air issue into the tube in the same time the mercury must rise near six inches, and so proportionably in other cases. Dr Hales observed, that the mercury rose highest, in most cases, when the sun was clear and warm, and that it subsided three or four inches towards evening, but rose again the next day as it grew warm, though seldom so high as at first. Dr Hales adapted the size and shape of the glass apparatus to a great variety of branches of several sizes and of different kinds of trees, and repeated the experiment above
Gage. above described, mutatis mutandis, in a variety of instances. See his Vegetable Statics, vol. i. chap. ii. p. 84, &c.