730

T II E

T II E

acquaints us with the rules that are to he observed, and the medi-cines to be employed, in the cure of diseases.

THERM,E, hot-baths or bagnios. Luxury and extravagancewere in nothing carried to such heights as in the thermal ol the Ro-man emperors.

THERMIA, an island in the Grecian Archipelago , so calledfrom its abounding with hot springs, south of the isle of Zia, nearthe Gulf of Engir. The soil is allowed to be fertile and well cul-tivated.

Thermia, an ancient city, capital of the above island, on the S.coast. I.on. 24. SO. E. Eat. 17. 31. N.

THERMOMETER, [Tr^o; and h'tjov,] an instrument formeasuring the degrees of increase or decrease of heat of the air, orany oilier bodies. The thermometer was invented about the be-ginning of the seventeenth century ; but, like many other usefulinventions, it has been found impossible to ascertain to whom thehonour of it belongs. Roerliaave ascribes it to Cornelius Drebbel of Alcm'aer, his own countryman . Fulgenzio attributes it to hismaster, Paul Sarpi , the great oracle of the Venetian republic; andViviani gives the honour of it to Galileo. But all these are post-humous claims. Sanctorio claims this honour to himself; andhis assertion is corroborated by Borelli and Malpighi, of the Flo-rentine academy, whose partiality is not to be suspected in favourof a member of the Patavinian school. The first form of this in-strument for measuring the degrees of heat and cold, was the air-thermometer. It is a well known fact, that air expands with heat,so as to occupy more space than it does when cold, and that it iscondensed by cold so as to occupy less space than when warmed,and that this expansion and condensation is greater or less accord-ing to the degree of heat or cold applied. The principle then onwhich the air-thermometer was constructed is very simple. Theair was confined in a tube by means of some coloured liquor; theliquor rose or fell according as the air became expanded or con-densed. What the first form of the tube was cannot now per-haps be well known, but the following description of the air-ther-mometer will fully explain its nature. The air-thermometer con-sists of a glass-tube, BE, (Plate GLXXIV. fig. I,) connected atone end with a large glass-ball, A, and at the oilier end immersedin an open vessel, or terminating in a ball, 1)E, with a narrow ori-fice at D ; which vessel or ball contains any coloured liquor thatwill not easily freeze. Aqua-fortis tinged of a fine blue colourwith a solution of vitriol or copper, or spirit of wine tinged withcochineal, will answer this purpose. But the hall A must he firstmoderately warmed, so that a part of the air contained in it maybe expelled through the orifice D ; ami then the liquor pressed bythe weight of the atmosphere will enter the ball DE, and rise, forexample, to the middle of the tube at C, at a mean temperatureof the weather; and in this state the liquor by its weight, and theair included in the ball A, &c. by its elasticity, will counterba-lance the weight of the atmosphere. As the surrounding air be-comes wanner, the air in the ball and upper part of the tube, ex-panding by heat, will drive the liquor into the lower ball, and con-sequently its surface will descend ; on the contrary, as the am-bient air becomes colder, that in the ball is condensed, and theliquor pressed by the weight of the atmosphere will ascend : so thatthe liquor in the tube will ascend or descend more or less accord-ing to the state of the air contiguous to the instrument. To thetube is affixed a scale of the same length, divided upwards anddownwards from the middle C into 100 equal parts, by means ofwhich the ascent and descent of the liquor in the tube, and con-sequently the variations in the cold or heat of the atmosphere,may be observed. This instrument was extremely defective;tor the air in the tube was not only affected by the heat and coldof the atmosphere, but also by its weight. The air being foundimproper for mcasutiug with accuracy the variations of heat andcold, different fluids at different times have been tried, and it hasbeen found that the best fluids for thermometers are either mer-cury or spirits of wine; the latter of which is generally tinged red,by means of cochineal, or Brazil wood, &c. for the purpose ofrendering it more visible ; hence they are denominated the nicrcu-rial-thermometer, and the spirit-thermometer. Other iluids, onaccount of their clamminess, or of their great irregularity of ex-pansion, are not useful for thermometers. The most proper andthe most useful shape for thermometers, is that of a long tube w ith2 narrow bore, and with globular cavity at one extremity, as

fig. 2. The cavity of the bulb C, and part of the tube, as far, forinstance, as A, are filled witlt the fluid: the rest of the tube iseither partly, or quite, exhausted of air; and the end B of thetube is hermetically sealed; viz. perfectly closed by melting theextremity of the tube at the (lame of a candle or lamp, urged bymeans of a blmv-pipe. When the bulb C is heated, the mercury',or the spirit of wine, is expanded; and not being aide to extenditself any other way, all the increment of bulk is manifested in thetube, viz. the surface A of the fluid will rise considerably into the,tube. On die other hand, when the Inilb G is cooled, the fluidcontracts, and its surface A descends, it is evident that, csterisparibus, the larger the hull) is, in proportion to the diameter ofthe cavity of the tube, or the narrower the latter is in proportionto the former, the greater will the motion of the surface A be inthe tube. But it must be observed, that when the bulb is verylarge, the thermometer will not easily arrive at the precise tem-perature of any place wherein it may he situated. Some persons*in order to give the bulb a greater surface, and of course to ren-der it more capable of readily attaining a given temperature, havemade it not globular, but cylindrical, (which shape was adoptedby Fahrenheit,) or flat, or bell-like, &c. but those shapes are im-proper, because they are liable to be altered by the varying gra-vity of the atmosphere, consequently those thermometers cannotbe accurate. If a thermometer is heated suddenly, as when thebulb C is immerged in hot water, the surface A of the fluid in it willbe seen to descend a little, and instantly after will be seen to rise ;the reason of which is, that the heat of the water enlarges the glassfirst, and is then communicated to the fluid, See. On the con-trary, if the bull) of a thermometer is cooled suddenly, the surfaceA of the fluid will first rise a little, and then will descend; because thecold contracts the glass alone at first, and afterwards contracts thefluid. Ice is melted by a certain invariable degree of temperature ;and water freezes at about the same temperature ; therefore, if thebulb C of a mercurial thermometer is placed in melting ice, ormelting snow, and a mark is made on the outside of the tube, evenwith the surface of the fluid, as at ID ; that mark is called thefreezing-point, though in fact it is the melting-point of ice; thefreezing-point of water being not so constant. If the bulb of thethermometer is placed in boiling water, and a mark is made on theglass-tube, even with the surface of the fluid within, as at E, thatmark is called the boiling-point; for in an open vessel, and underthe same atmospherical pressure, which is indicated by the baro-meter, water constantly boils at the same temperature/ and an in-creased lire will force it to evaporate faster, but will not raise itstemperature. Those points being ascertained, if the length of thetube from D to E is divided into any number of equal parts,those parts will be the degrees of the thermometer, or the degreesof heat, indicated by the corresponding expansions of the fluidwithin the thermometer. And tlu; same degrees, or equal divi-sions, may be continued below D and above E, in order to shewthe degrees of temperature below the freezing, and above the boil-ing, point. Those two unalterable points of temperature, viz. theformer where ice becomes water, and the second where water be-comes vapour, have been universally adopted by the various con-structors of thermometers for the graduation of those instruments;hut the space between them has been divided differently by dif-ferent persons, and this difference gives the different names ofthennometers, or rather of their graduations; such as Reaumur’sthermometer, Fahrenheit’s thermometer. See. Reaumur dividesthe space between the above-mentioned two points into 80 equalparts or degrees; placing the 0 at freezing, and the 80th degree atthe boiliiig-poili 1 . Fahrenheit divides it into 180 degrees or equalparts, but he places the 0 thirty two degrees below the freezing-point D ; so tlut the freezing-point is at 32, and the boiling-pointE is at 212 degrees, Ollier persons have adopted other divisions,which have been suggested by supposed advantages or fancifulideas. Most of those graduations are at present out of use, butthey are to be met with in various, not very recent, publications;we have therefore thought it necessary to set them down in thefollowing table, which contains: 1st, The name of the poison orsociety that has used each particular division: 2dly, The degreewhich has been placed, by each of them, against the freezing-point ; 3dly, The degree which has been placed against the boiling-point: and, 4thly, The number of degrees lying between thosetwo points.

Fahrenheit’s*