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twice the length of the other. Then introduceinto the longer tube a little fulphuric acidtinged with carmine, fufficient to iill about aninch of its cavity, join the two tubes togetherby the blow-pipe, and when joined, bendthem in the form of the letter U. with thebulbs about 3 or 4 inches afunder, making•one flexure juft below the juncture of thetwo tubes, where the fmall cavity (which isreprefented in the plate] facilitates the adjuft-ment of the inftrument, which by a little dex-terity is performed by forcing a few globules ofair by the heat of the hand from one bulb tothe other. Attach a graduated fcale to the■fhorter tube, making the zero about the middle■of it, and adjuft to it the quantity of air in eachbulb, fo that when the bulbs are at the fametemperature, the upper furface of the colouredliquor may juft correfpond with the zero.Sulphuric acid is chofen as the liquor interpofedbetween the bulb, on account of its bearing•any heat or cold that 'would be ufed withoutbeing evaporated or congealed.

I11 this inftrument the air inclofed in the•bulbs is the fubftance which, by its expanfion-or contraction, caufes the motion of the colouredliquor up or down the fcale, and as gaffes aremuch more expanfible than liquids, the inftru-ment is fooner affeCted by minute changes ofheat. But as the two bulbs are of equal fize,and both filled with air, and feparated fromeach other by the intervening liquor, it is ob-vious that when the temperature is the fame in•each bulb, be it high or low, the preffure oneach fide of the liquor is alfo equal, and it muft' remain ftationary: fo that it can only move'when one bulb is warmer than the other.Hence the particular and foie ufe of this inftru-ment as a Differential Thermometer. The lowerpart of the inftrument (or the fpace included■ between the two bends) is cemented to an'- upright ftem by which it is fupported.

TROUGH. Pneumatic.

In all the common operations of receivingand transferring gaffes they are held in jars, orother veflels, previoufly filled with water, andinverted over this fluid. It is therefore neceffary' to have fome kind of water-veffel large and deepenough entirely to immerfe the air-jar, thatevery particle of common air may be difplacedby the water before it receives the gas to be••operated on. Dr. Prieftley’s firft water troughfor this purpofe was a common oval or roundtub, but as a jar full of air ftanding invertedover water and immerfed in it for any depth,would readiiy be overfet by the preflurc of the

water, this ingenious philofopher adapted ftfmall hanging fhelf fufpended from the fides ofthe tub, and finking below the level of thewater, only juft fufficient to confine the gas inany jar ftanding upon it. This plan has beenadopted with fome little improvements in thep eumatic water troughs commonly ufed, ofwhich Fig. 49. PI. IV. is one of the moltconvenient kinds, a is a deep oblong troughmade of thin tinned iron well japanned, ge-nerally 18 inches long, 9 broad, and 14 deep.About 3I inches from the top is a fhelf of thefame material, extending entirely acrofs thetrough, and rather more than a third of itslength, and fixed in its fituation when requiredby two ftrong wires. This fhelf is given in-verted in Fig. 50. to fhew an oval rim pro-jecting from it, in the center of which are twofmall holes, the ufe of which is to fecure andconvey the gas into any veflel fet upon it (asthe bottle in Fig. 49). This fhelf has alfo twolarger holes to receive two bottle fupporters,Fig. 51. & A Fig. 49. A trough of this fizeand conftruCtion is very light when empty, andlarge enough for rnoft operations.

Fig. 52. is a convenient trough for exhibitingexperiments in the leCture room. It is Amplya circular pan, 18 inches in diameter, with aloofe femicircular fhelf fupported to the requi-fite height on three conical legs, on which abell receiver is reprefented ftanding. This panis however too fhallow to allow of any largeveflel to be immerfed, fo that it muft be pre-- vioufly immerfed in a deeper trough, and con-veyed hither upon a plate or faucer.

Fig. 53. is a Mercurial Pneumatic Trough,which is indifpenfible in molt experiments ongaffes abforbahle by water. It may be madeeither of a clofe hard wood fuch as mahogany,or cut out of a block of ordinary marble or free-ftone, or as is now more common, it is a thiniron cafe inclofed in wood. When made ofwood it is often hollowed out of a Angle piece,but if grooved and very neatly rabitted, it willanfwer nearly, as well. In any cafe it fhouldalfo ftand on a fhallow wooden or iron difh tocatch the mercury, fome of which is alwaysfpiiling whatever care be ufed.

Thefe mercurial troughs muft be made veryfolid to bear the great weight of metal that isrequired even for a very moderate fized veflel,and for the fame reafon and to fave the greatexpencp of the metal, they are always mademuch fmaller than the water troughs. Thiswill not appear an indifferent objedt when it isconfidered that half a hundred weight of this