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ftatically, or elfe lie fhould be provided with anhydrometer, of which Mr.Nicholfon’s (or that inwhich the point of immerfion is fixed)is byfarthebeft. The fpecific gravity of fluids may alfo bevery conveniently taken in the following way:take a fmall light bottle that Hands firmly, andholds an ounce or two, or more, of water, theneck of which is Hopped by a piece of baro-meter tube very accurately ground. Thencounterpoife the empty bottle, and afterwards fillit entirely with diftilled water at 6o° recentlyboiled, tiil it rifes a little in the bore of thetube and find the weight. For greater eafe incalculation it w'ill be as well (if it can be done)to bring the water exaftly to that height in thetube, at which its weight will be 500 grains,or 1000, or 1500, or 2000, &c. but this is noteflential. Then mark the height mofi accu-rately with a fine file on two oppofite fides ofthe tube, and fcratcli on the bottle the weightboth of the bottle and tube when empty, andof the quantity of-water requifite to fill it tothe mark, This will then ferve for the fpecificgravity of all other fluids, as pure water is theHandard of comparifon for all bodies folid aswell as fluid.

STILL. See Distilling Apparatus.

THERMOMETER . Several varieties ofthermometer are made for philofophical pur-pofes, and in many experiments, particularly allthofe connected with the fubjeft of caloric, itis necefiary to be well provided with them. Totake in the entire range of thermometrical tem-perature from the moH intenfe artificial cold tothe boiling point of mercury requires a verylong tube -, but for moH chemical purpofes it isfufficiently high if it is graduated to about 10degrees above the boiling point, which willreach the temperature of mofi faline folutionswhen boiling.

For experiments on intenfe cold, it is necef-fary to have a fpirit thermometer graduatedabout io®°. below «. and the lower extremityof the fcale fhould be at feme difiance fromtl.e bulb, that the temperature may be obfervedwithout lifting the bulb out of arty deep veffelthat may contain the freezing mixture Themofi delicate and fenfible thermometers aremade with a very fmall bulb, fcarcely largerthan the Item, and a tube of an extremely nar-row bore, not larger than a horfe-hair. Forchemical purpofes alfo the fcale fhould either befcratched on the glafs itfelf, or as. this is diffi-cult to be feen- in a common light, an ivoryfcale fhould be attached, without reaching fo

low as the bulb, that the latter may be falelyimmerfed in acid or corrofive liquors.

The thermometer fcale has been differentlydivided by the original conftrudlors, whichoften caufes a good deal of trouble in compa-ring refults of experiments made in differentcountries. Three feales are principally ufed:Fahrenheit’s, which is employed in this country,in which the o. is 32 degrees below the freezingpoint of water, and the interval between o. andthe boiling, point of water is divided into 212degrees, and confequently the freezing point ofwater is 32° : Reaumur’s, which has the o. at thefreezing point of water, and divides the inter-val between this point and the water-boilingheat into 80 degrees : and Celfius’s, originallyufed in Sweden, and fince adopted in France under the name of the Centigrade , which has,like Reaumur’s, the ©. at the freezing point ofwater, but divides the interval between this andwater-boiling heat into 1 eo degrees. The rcafonwhy Fahrenheit fixed his fcale fo far below thewater-freezing point was founded on an erro-neous hypothefis relative to the real zero orpoint of abfolute privation of heat •, but howeverit has this advantage over the two others, whichis, that the diftinction between the pofitiveand negative terms (or which exprefs degreesabove or below the zero) much lefs frequentlyoccurs in any experiments, and fcarcely everin the regiffer of natural cold in temperate cli-mates, by which many accidental errors areavoided.

In Appendix II . we have given the corref.ponding degrees of thofe three thermometers,,by which the reader may at once tranflate thedegrees of Reaumur’s and Celfius’s into theequivalent of Fahrenheit, or vice verfa,

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A very Ingenious thermometer has been in-vented by Mr. Leflie, which be calls a Differ-ential Thermometer, as it expreffes not theabfolute degree of heat, but the difference(when any exiffs) between- the temperature ofthe two fpots where its two bulbs are placed.Its life is explained under the article Caloric.

The differential thermometer (PI. IV. Fig.54.) is made in the following way: feleft two-thermometer tubes with bores rather widerthan ufual, and one a little wider than the other.Let the balls be blown as equal as the eyecan judge and from .4 to .7 of an inch in dia-meter, and let the open end of the tube alfo bewidened in a flight degree. The tubes nraft beof unequal length, the longeft being nearly