, in Hydrostatics, Pneumatics, &c, is an instrument for ascertaining measures of various kinds. As

Gage of the Air-pump, is adapted for shewing the degree to which the air is rarefied, or the receiver is exhausted, at any time by the air-pump. This is either the common barometer-gage, both long and short, or the pear gage, which at first was thought a great improvement, but afterwards it was discovered that its seeming accuracy was founded on a fallacy, which gave an erroneous indication of exhaustion. See Air-pump.

Gage of the Barometer, is a contrivance for estimating the exact degree of the rise and fall of the mercury in the tube of that instrument. It is well known that whilst the mercury rises in the tube, it sinks in the cistern, and vice versa; and consequently the divisions on the scale fixed near the top of the tube had their distance from the surface of the mercury in the cistern always various; from which there must often happen errors in determining the height of the mercury in the tube. To remedy this inconvenience, a line is cut upon a round piece of ivory, which is fixed near the cistern: this line is accurately placed at a given distance from the scale; for example at 27 inches; and a small float of cork, with a cylindrical piece of ivory fixed to its upper surface, on which a line is cut at the exact distance of 2 inches from the under side of the cork, is left to play freely on the quicksilver, and the cylinder works in a groove made in the other piece. From this construction it appears, that if these marks are made to coincide, by raising or lowering the screw which acts on the quicksilver, then the divisions on the scale will express the true measure of the distance from the surface.

Gage of the Condenser, is a glass tube of a particular construction, adapted to the condensing engine, and de- <*>gned to shew the exact density and quantity of the air contained at any time in the condenser. See Desaguliers's Exper. Philos. vol. 2, p. 394.

Sea Gage, an instrument for finding the depth of the sea. Several sorts of these have been invented by Dr. Hales, Dr. Desaguliers, and others. Formerly, the machines for this purpose consisted of two bodies, the one specisically lighter, and the other specifically heavier than the water, so joined together, that as soon as the heavy one came to the bottom, the lighter should get loose from it, and emerge; and the depth was to be estimated by the time the compound was in falling from the top to the bottom of the water, together with the time the lighter body was in rising, reckoned from the disappearing of the machine, till the emergent body was seen again; but no certain conclusion could be drawn from so precarious and incomplete an experiment.

But that invented by Drs. Hales and Desaguliers was of a more exact nature, depending on the pressure of the fluid only. For as the pressure of fluids in all directions is the same at the same depth, a Gage which discovers what the pressure is at the bottom of the sea, will shew what the true depth of the sea is in that place, whether the time of the machine's descent be longer or shorter.

Dr. Hales, in his Vegetable Statics, describes his Gage for estimating the pressures made in opaque vessels; where honey being poured over the surface of mercury in an open vessel, rises upon the surface of the mercury as it is pressed up into a tube whose lower orifice is immersed into the honey and mercury, and whose top is hermetically sealed. Now as by the pressure, the air in the tube is condensed, and the mercury rises, so the mercury comes down again when the pressure is taken off, and would leave no mark of the height to which it had risen; but the honey (or treacle, which does better) which is upon the mercury, sticking to the inside of the tube, leaves a mark, which shews the height to which it had risen, and consequently gives the quantity of pressure, and the height of the surface of the fluid.

Desaguliers's addition to this machine, consisted in a contrivance to carry it down to the bottom of the sea by means of a heavy weight, which was immediately disengaged by striking the bottom, and the Gage, made very light for the purpose, re-ascended to the top.

Dr. Hales afterwards made more experiments of this sort, and proposed another Sea Gage for vast depths, which is described in the Philos. Trans. N° 405, and is to this effect. Suppose a pretty long tube of copper or iron, close at the upper end, to be let down into the| sea, to any depth, the water will rise in the tube to a height bearing a certain proportion to the depth of the sea to which the machine is sunk. And this proportion is as follows: 33 feet of sea water being nearly equal to the mean pressure of the atmosphere, therefore at 33 feet deep, the air in the tube will be compressed into half the length of the tube, or the water will rise and fill half way up the tube; in like manner at 66 feet deep, the water will occupy 2/3 of the tube; at 99 feet deep it will fill 3/4 of the tube; at 132 feet de<*>p it will fill 4/5 of the tube; and so on. Hence therefore, by knowing the height to which the water rises in the tube, there will be known the consequent depth of the sea.

But, in very great depths, the scale near the top of the tube would be so small, and the divisions so close, that there would be no accuracy in the experiment, unless the tube were of a very great length, and this again would render it both liable to be broken, and quite impracticable.

To remedy this inconvenience, he made the following contrivance: To the bottom of the tube he screwed a large hollow globe of copper, with a small orisice, or a short pipe at bottom of the globe, to let in the water; by which means he had a very great quantity of air, and the scale enlarged. See also Desagul. Exp. Phil. vol. 2, p. 224 and 241.

Bucket Sea Gage, is an instrument contrived by Dr. Hales to find the different degrees of coolness and saltness of the sea at different depths. This Gage consists of a common pale or bucket, with two heads: these heads have each a round hole in the middle, about 4 inches in diameter, covered with square valves opening upward; and that they may both open and shut together, there is a small iron rod, having one end fixed to the upper side 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 end of the bucket; so that the bucket is drawn up full of the lowest sea water to which it has descended, and immediately the mercurial thermometer, fixed within it, is examined, to see the degree of temperature; and the degree of saltness is afterwards examined at leisure. Philos. Trans. numb. 9, p. 149, and numb. 24, p. 447, or Abridg. vol. 2, p. 260.

Lord Charles Cavendish adapted a thermometer for the temperature of the sea water, at different depths. See Philos. Trans. vol. 50, p. 300, and Phipps's Voyage towards the North Pole, p. 142 &c.

Aqueo-mercurial Gage is the name of an apparatus contrived by Dr. Hales, and applied, in various forms, to the branches of trees, to determine the force with which they imbibe moifture. Vegetable Statics, vol. 1, ch. 2, p. 84.

Sliding Gage, a tool used by mathematical instrument makers, for measuring and fetting off distances; consisting of a beam, tooth, sliding socket, and the shoulder of the socket.

Tide Gage, 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 the years 1772, 1773, 1774, and 1775. This instrument consists of a glass tube, whose internal diameter was 7-10ths of an inch, lashed fast to a 10 foot fir rod, divided into feet, inches, and parts; the rod being fastened to a strong post fixed firm and upright 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 the 10th part of an inch when the swell of the sea was 2 feet; and Mr. Bayly was certain, that with this instrument he could discern a difference of the 10th of an inch in the height of the tide.

Water Gage. See Altitude, and Hydrometer.

Wind Gage, an instrument for measuring the force of the wind upon any given surface. Several have been invented formerly, and one was lately invented by Dr. Lind, which is described in the Philos. Trans. vol. 65. See several also under the article Anemometer.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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