Sea-GAGE — 3rd ed. (1797)

Sea-GAGE, an instrument invented by Dr Hales and Dr Defagulier for finding the depth of the sea; the description whereof is this. AB (Plate CCV. fig. 1. n^o 1.) is the gage-bottle, in which is cemented the gage-tube F 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 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 pocket N, with a notch on one side at m, in which is fixed the catch l of the spring S, and, passing

Gage. 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 buoyoys 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 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. n^o 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 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 Fig. 2. 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 3900 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 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 E, E, 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 is 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

Gage. cluded water is kept from getting out, and the ambient water kept from getting in. Phil. Trans. N^o 9. p. 149. and N^o 24. 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 cr, 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 $\frac{1}{4}$ 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 a 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 ze is cemented, being about $\frac{1}{4}$ of an inch diameter, and 18 inches long, and in substance full $\frac{1}{4}$ of an inch thick. These tubes are cemented together at e with common hard brick-dust or powdered chalk cement, 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 ze from the cistern x, so that in half an hour it was risen $5\frac{1}{2}$ inches high, as far as z. 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 ze, 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 it 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 described, mutatis mutandis, in a variety of instances. See his Vegetable Statics, vol. i. chap. ii. p. 84, &c.

Tide-Gage. is the name of an instrument used for determining the height of the tides by Mr Bayly, in the course of a voyage towards the south-pole, &c. in the Resolution and Adventure, in 1772, 1773, 1774, and 1775. This instrument consists of a glass tube, whose internal diameter was seven-tenths of an inch, lashed fast to a ten feet fir rod, divided into feet, inches, and quarters: this rod was fastened to a strong post fixed upright and firm in the water. At the lower end of the tube was an exceeding small aperture, through which the water was admitted. In consequence of this construction, the surface of the water in the tube was so little affected by the agitation of the sea, that its height was not altered one-tenth of an inch, when the swell of the sea was two feet; and Mr Bayly was certain, that with this instrument he could discern a difference of one-tenth of an inch in the height of the tide.

Wind-Gage. an instrument for measuring the force of the wind upon any given surface. It was invented by Dr Linn, who gives the following description of it, Phil. Trans. Vol. LXV.

This instrument consists of two glass tubes AB, CD, of five or six inches in length. Their bores, which are so much the better for being equal, are about fourths of an inch in diameter. They are connected together like a siphon, by a small bent glass-tube a b, the bore of which is about one-tenth of an inch in diameter. On the upper end of the leg AB there is a tube of latten brass, which is kneed, or bent perpendicularly outwards, and has its mouth open towards F. On the other leg CD, is a cover with a round hole G in the upper part of it, two-tenths of an inch in diameter. This cover and the kneed tube are connected together by a slip of brass ed, which not only gives strength to the whole instrument, but also serves to hold the scale HI. The kneed tube and cover, are fixed on with hard cement or sealing wax. To the same tube is soldered a piece of brass e, with a round hole in it to receive the steel spindle KL; and at f there is just such another piece of brass soldered to the brass-hoop gb, which surrounds both legs of the instrument. There is a small shoulder on the spindle at f, upon which the instrument rests, and a small nut at i, to prevent it from being blown off the spindle by the wind. The whole instrument is easily turned round upon the spindle by the wind, so as always to present the mouth of the kneed tube towards it. The end of the spindle has a screw on it; by which it may be screwed into the top of a post or a stand made on purpose. It has also a hole at L, to admit a small lever for screwing it into wood with more readiness and facility. A thin plate of brass k is soldered to the kneed tube, about half an inch above the round hole G, so as to prevent rain from falling into it. There is likewise a crooked tube AB (fig. 5.) to be put occasionally upon the mouth of the kneed tube F, in order to prevent rain from being blown into the mouth of the wind-gage when it is left out all night, or exposed in the time of rain.

The force or momentum of the wind may be ascertained by the assistance of this instrument, by filling the tubes half full of water, and pushing the scale a little up or down, till the o of the scale, when the instrument is held up perpendicularly, be on a line with the

Gage. the surface of the water in both legs of the wind-gage. The instrument being thus adjusted, hold it up perpendicularly, and turning the mouth of the knee tube towards the wind, observe how much the water is depressed by it in the one leg, and raised in the other. The sum of the two is the height of a column of water, which the wind is capable of sustaining at that time; and every body that is opposed to that wind will be pressed upon by a force equal to the weight of a column of water, having its base equal to the altitude of the column of water sustained by the wind in the wind-gage. Hence the force of the wind upon any body where the surface opposed to it is known, may be easily found; and a ready comparison may be made betwixt the strength of one gale of wind and that of another.

The force of the wind may be likewise measured with this instrument, by filling it until the water runs out at the hole G. For if we then hold it up to the wind as before, a quantity of water will be blown out; and if both legs of the instrument are of the same bore, the height of the column sustained will be equal to double the column of water in either leg, or the sum of what is wanting in both legs. But if the legs are of unequal bores, neither of these will give the true height of the column of water which the wind sustained. But the true height may be obtained by the following formula.

Suppose that after a gale of wind which had blown the water from A to B (fig. 6.), forcing it at the same time through the other tube out at E, the surface of the water should be found standing at some level DG, and it were required to know what was the height of the column EF or AB, which the wind sustained. In order to obtain this, it is only necessary to find the height of the columns DB or GF, which are constantly equal to one another; for either of these added to one of the equal columns AD, EG, will give the true height of the column of water which the wind sustained.

1. Let the diameters AC, EH, of the tubes, be respectively represented by $cd$ ; and let $a = AD$ , or $EG$ , and $x = DB$ , or $GF$ : Then it is evident, that the column DB is to the column EG, as $c^2x$ to $d^2a$ . But these columns are equal. Therefore $c^2x = d^2a$ ; and consequently $x = \frac{d^2a}{c^2}$ .

2. But if at any instant of time whilst the wind was blowing, it was observed, that, when the water stood at E, the top of the tube out of which it is forced, it was depressed in the other to some given level BF, the altitude at which it would have stood in each had it immediately subsided, may be found in the following manner. — Let $b = AB$ or $EF$ . — Then it is evident that the column DB is equal to the difference of columns EF, GF. But the difference of these columns is $2b - d^2x$ ; and consequently $x = \frac{d^2b}{c^2 + d^2}$ .

For the cases when the wind blows in at the narrow leg of the instrument: Let $AB = EF = b$ , $EG$ , or $AD = a$ , $GF = DB = x$ , and the diameters $EH$ , $GA$ , respectively $= d$ , $c$ , as before. Then it is evident, that the column AD is to the column GF as $ac^2$ to $a^2c$ . But these columns are equal; therefore $d^2x = ac^2$ ;

and consequently $x = \frac{ac^2}{d^2}$ . It is also evident, that the column AD is equal to the difference of the columns AB, DB; but the difference of these columns is as $bc^2 - c^2x$ . Therefore $d^2x = bc^2 - c^2x$ . Whence we get $x = \frac{bc^2}{d^2 + c^2}$ .

The use of the small tube of communication $a b$ (fig. 4.), is to check the undulation of the water, so that the height of it may be read off from the scale with ease and certainty. But it is particularly designed to prevent the water from being thrown up to a much greater or less altitude, than the true height of the column which the wind is able at that time to sustain, from its receiving a sudden impulse whilst it is vibrating either in its ascent or descent. As in some cases the water in this instrument might be liable to freeze, and thus break the tubes, Dr Lind recommends a saturated solution of sea-salt to be used instead of it, which does not freeze till Fahrenheit's thermometer falls to 0.