on. On the top, in the upper part of the brass tube at II, is fixeda large empty ball, or full blown bladder I, which must not be solarge but that the weight K may be able to sink the whole underwater. The instrument thus constructed is used in the followingWanner. The weight K being hung on, the gage is let fall intodeep water, and sinks to the bottom ; the socket N is somewhatlonger than the shank L ; and therefore, after the weight K comesto tbe bottom, the gage will continue to descend till the lowerpart of the socket strikes against the weight; this gives liberty tothe catch to fly out of the hole L, and let go the weight K: whenthis is done, the ball or bladder I instantly buoys up the gage tothe top of the water. While the gage is under water, the waterpaving free access to the treacle and mercury in the bottle, will by'ts pressure force it up into the tube Vf, and the height to which ithas been forced by the greatest pressure, viz. that at the bottom, willhe shewn by the mark in the tube which the treacle leaves behindif, and which is the only use of the treacle. This shews into whatspace the whole air in the tube F/is compressed, and consequent-ly the height or depth of the water which by its weight producedfhat compression, which is the thing required. If the gage-tube1/ be of glass, a scale might be drawn on it with the point of adiamond, shewing, by inspection, what height the water standsshove the bottom. But the length of ten inches is not sufficientf°r fathoming depths at sea, since that, when all the air in such alength of tube is compressed into half an inch, the depth of waterno more than 634 feet, which is not lull fa quarter of a mile. If,remedy this, we make use of a tube fifty inches tong, which forstrength may be a musket-barrel, and suppose the air compressed'nto a hundredth part of half an inch ; then by say ing, 1 : 9ft : : 400: 3ytiu0 undies, or 3300 feet; even this is but little more than half a1! file, or 0640 feet. But since it is reasonable to suppose the ca-tties of the sea bear some proportion to the mountainous parts offhe land, some of which are more than three miles above theOrth’s surface ; therefore, to explore such great depths, the doctoryoutiived a new form for his sea-gage, or rather for the gage-tube! n it, as follows: BCDF, fig. 4, is a hollow metalline globe com-municating on the top with a long lube, AB, w hose capacity is ab'nth part of that globe. On the lower part, at D, it has also a_ °rt tube DE, to stand in the mercury and treacle. The airc °ntained in the compound gage-tube is compressed by the waterj 18 before; but the degree of compression, or height to which the^-‘acle has been forced, cannot there be seen through the tube ;therefore, to answer that end, a slender rod of metal or wood, with* unob on the lop of the tube AB, will receive the mark of theqeaele, and shew it when taken out. If the tube AB be fiftyll 'fhes, and of such a bore that everv inch in length should be a^uuic inch of air, and the contents of the globe and lube together, 0 cubic inches; then when the air is compressed within a liun-^•'edth part of the whole, it is evident the treacle will not approachfarer than five inches of the top of the tube, which will agree0 the depth of 3300 feet of water as above. Twice this depth will'empress the air into half that space nearly, viz. two inches and awhich correspond to 6600, which is a mile and a quarter.

"Sain, half that space, or one inch and a quarter, will shew

^mible the former depth, viz. 13200 feet, or two miles and a half;^'ch is probably very nearly the greatest depth of the sea.'•’age, vVind, an instrument for measuring the force of thebid upon any given surface. It was invented by Dr. Lind, whotlb'es the following description of it, Philosophical 1 ransactions,0 f - '*v. See Plate LXXVI11. fig. 5. This instrument consists-r,, lw ° glass-tubes, AB, CD, of five or six inches in length.

eir bores, which are so much the better for being equal, are

t 01,1 four-tenths of an inch in diameter. They are connected

^..^ydier like a siphon, by a small bent glass-tube, a, b, the bore ofi is about one-tenth of an inch in diameter. On the upperof the leg All there is a tube of latten brass, which is kneed,bent perpendicularly outwards, and has its mouth open to-artls p. Qu the otlier u , g CD is a cover with a round holeu > the upper part cf it, tw o-tenths of an inch in diameter. Thish” 7 ai '(l the kneed tube are connected together by a slip ofh.\ s ' e d, which not only gives strength to the whole instrument,also serves to hold the scale III. The kneed tube and covertul 011 with hard cement, or sealing-wax. to the same

18 s °ldcretl a piece of brass e, with a round hole m it to re-■ vt th « steel spindle KL ; and at/ there isjust another piece of

°l.. It.- so yq

brass soldered to the brass hoop gh, which surrounds both legs ofthe instrument. Theie is a small shoulder on the spindle at/, upon which the instrument rests, and a small nut at i, to pre-vent it from being blown off the spindle by the wind. The wholeinstrument 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 lias a screw on it; by which it may bescrewed into the top of a post or a stand made on purpose. It hasalso a hole at L, to admit a small lever for screwing it into woodwith more readiness and facility. A thin plate of brass Ay is sol-dered to the kneed tube, about half an inch round th.- ’hole G, soas to prevent rain from falling into it. There is likewise a crookedtube AB, fig 6, to be put occasionally upon the mouth of thekneed tube F, to prevent rain from being blown into the monthof the wind-gage when it is left out ail night, or exposed in thetime of rain. The force or momentum of the wind may be ascer-tained by this instrument, by filling the tubes half full of water,and pushing the scale a little up or down, till the 0 of the scale,when the instrument is held up perpendicularly, be on a line withthe surface of the water in both legs of the wind-gage. The in-strument being thus adjusted, hold it up perpendicularly, andturning the mouth of the kneed tube towards the wind, observehow much the water is depressed by it in the one leg, and raisedin the other. The sum of the two is the height of a column ofwater which the wind is capable of sustaining at that time; andevery body that is opposed to that w ind will he pressed upon bya force equal to the weight of a column of water having its baseequal to the altitude of the column of water sustained by thewind in the wind-gage, lienee the force of the wind uponany body, where the surface opposed to it is known, may be easily-found, and a ready comparison may be made betwixt the strengthof one gale of wind and that of another. The force of the windmay likewise be measured with this instrument, by filling it untilthe water runs out at the hole G. For if we then hold it up tothe wind as before, a quantity of water will be blown out; and ifboth legs of the instrument are of the same bore, the height of thecolumn sustained will be equal to double the column of water ineither leg, or the sum of w hat is wanting in botn legs. But if thelegs are of unequal bores, neither of these will give the true heightof the column of water which the wind sustained. But the trueheight may be obtained by the following formulae. Suppose thatafter a gale of wind which had blown the water from A to B, fig.7, forcing it at the same time through the other tube out at E, thesurface of the water should be found standing at some level DG,,and it were required to know what was the height of the columnEF or AB, which the wind sustained. In order to obtain this, itis only necessary to find the height of the columns DB or GF,which are constantly equal to one another; for either of theseadded to one of the equal columns AD, EG, will give the trueheight of the column of water which the wind sustained. I. Letthe diameters AC ,EII, of the tubes be respectfully representedby c d\ and let a = AD, or EG, and x = DB, or GF : then it isevident that the column DB is to the column EG, as c-j to d-a.But these columns are equal. Therefore c-x =: d'a ; and conse-

d-tt.

qaenlly x — --. II. But if at any instant of time whilst the

c 2

wind was blowing, it was observed, that, when the water stood atE, the top of the tube out of which it is forced, it was depressedin the other to some given level BF, the altitude at which it wouldhave stood in each, had it immediately subsided, may be found inthe following manner: Let b = AB or EF. Then it is evidentthat the column DB is equal to the difi'erence of columns EF,GF: But the difference of these columns is as d q b—d-x ; andd-b

consequently x =-. For the cases when the wind blows

c 5 + d 2

in at the narrow leg of the instrument: Let AB — EF = b, EG,or AD = a, GF = DB =: x, and the diameters EH, GA, respec-tively =s d, c f as before, Then it is evident that the column ADis to the column GE as ac" to d-x. But the columns are equal;

uc 1

therefore d*x — nc 2 ; and consequently x —-. It is also evi-

d-

dent that the column AD is equal to the difference of the columns6S AB.