Thermometer, or Thermoscofe, an instrument for measuring, with accuracy, the degrees of heat or cold in the air, water, etc. - It is probable, that the expansion and contraction of bodies, in consequence of heat and cold, afforded the first idea for the contrivance of this useful scale.
The thermometer was invented early in the 17th century ; and, on account of its extensive service in the arts, manufactures, and domestic life, the original merit of constructing this valuable instrument is variously attributed to Cornelius Drebbel, Paul Sarpi, Galileo, and Sanctorio; though, we believe, the person first men. tioned, deserves the credit of having invented the earliest thermo-scope, in the year 1638, at Alk-maar, in North Holland. It was, however, an imperfect attempt; being regulated solely by the expansion of the air ; and serving at the same time as a barometer ; for his tube contained a mixture of water and aqua-fortis. Hence it was, in its primary state, called an air-thermometer; and, after substituting various other liquids, which were adopted by the Florentine Academy, by Sir Isaac Newton, and other philosophers, but which, on account of their uncertain properties, have long been disused, Dr. Halley proposed, in the year l680, to employ quicksilver for this important purpose. Daniel Gabriel Fahrenheit, a native of Danzig, was the first who successfully availed himself of that suggestion, in 1709 ; when he produced his newly graduated tube, containing the mercurial fluid. - He ascertained the artificial freezing point in the same year, during the severe winter; by observing, that a mixture of snow and sal-ammoniac, uniformly caused spirit of wine to descend to the point marked, and no farther. - Hence, he was induced to denote such part of his cylindrical tube with O or zero : from this point to that of the usual temperature of the human body in a healthy state, he divided the intermediate space into 96 degrees 5 and thence to that of boiling water into 126°, so that the latter mark, in his scale, rises exactly to 212°. Thus, Fahrenheit claims not only the merit of having Introduced the most scientific and convenient division of the thermometrical scale, but also that of adopting the mercurial fluid, which is doubtless the most proper, and perhaps the only one furnished by Nature, for measuring the various degrees of heat and cold, in the most accurate and sensible manner. - M. De Luc has proved, by conclusive arguments, the superiority of thermometers supplied with mercury, over those containing alkohol; and that the rising of the mercury, at all seasons, very nearly corresponds with the increase of heat. On the other hand, De Serviere remarked, that the column of mercury, in a perpendicular thermometer, tends to impede the accurate motion of this metallic fluid, on account of its own specific gravity : hence, he has judiciously proposed to remedy such defect, by keeping the instrument in a horizontal position. M. De Reaumur, in 1730, proposed a new division of the thermometrical scale, by introducing 1000 parts of diluted spirits of wine into his tube with the usual globe, then commencing to count from 0 upwards, to the boiling point, which he marked with 80°. Such contrivance, however, having been found inconvenient for calculating the different degrees of heat and cold, with sufficient accuracy, Reaumur's globes, with their lubes, were subsequently also furnished with mercury 5 and are at present chiefly employed by the French, and in some parts of Germany. - To enable the reader to form a distinct idea of the difference subsisting between the two instruments, here described, we shall subjoin a comparative table of their scales.
From this comparative view, it will appear that nine degrees of Fahrenheit's (between the point of natural freezing, and that of boiling heat), are equal to four of Reaumur's computation : - hence the scale of the former may be reduced to that of the latter, by multiplying the number of degrees above 32 (because 32 must be sub-tracted from the whole number given) by 4, and dividing the pro-duct by 9. - On the other hand, if the given degrees be multiplied by 0, and the product divided by 4, Reaumur's scale may be reduced to that of Fahrenheit. - Upon these principles, the preceding tables are calculated, with two decimals only below the freezing point. But, if any intermediate degrees be wanted, they may be easily ascertained, by a similar arithmetical process.
It would be. needless to describe the various thermometers invented during the last century; because the two instruments before mentioned have received almost universal sanction; few others being at present employed, either by artists or men of science. Hence we shall merely observe, that De L'Isle, at Petersburgh, contrived a new thermometer, in 1733, which is divided into 50°. - Prof. Christin, of Lyons, proposed a scale of 100 equal parts between the freezing and boiling points : this division was adopted by Prof. Celsius, in Upsala, whose improved scale, consisting of 100 exact degrees, is generally used in Sweden.
There are many other thermometers, among which, those by DucRest of Geneva ; by Stroh-meyer and BraNder, in Germany ; by Grubert in France (who, together with the three preceding, contrived the Universal Thermo-meter, which contains the comparative scales of 28 different instruments); by De Luc; Renaldi-NI; Cavendish; Konigsdorfer; Mudge; the Bernoullis; Fontana; Achard; Lichtenberg; Landriani ; Saussure; CasTelli ; And Luz.
The first idea for constructing a metallic thermometer, or properly, pyrometer, was furnished by Graham, and improved upon by MoR-timer, Ingham, Count Loser, Fitzgerald, Felter, and at length brought to perfection by Wedgewood. This ingenious nufacturer (of the finest earthenware from basaltic masses, or terra-cotta; has rendered an essential service to society, by his method of ascertaining high degrees of heat: for this purpose he employs small cubes of dry clay; because that species of earth has the remarkable property of contracting in its bulk, when submitted to the fire, and not again expanding, on suddenly exposing it to the cold air. In order to ascertain the precise degree of heat in an oven, he puts one of his clay-cubes into it; and, after having acquired the temperature of the place, he immediately plunges It into cold water. Now, the size of the cube (that was exactly adjusted to half an inch square) is measured between two brass rules, the sides of which are somewhat obliquely disposed, so as to form an irregular groove, into which the cube may be slidden. In proportion as the bulk of the latter has been contracted by heat, it passes down deeper between the scales, on which the various degrees of temperature have been previously marked. Thus, when the division of the scale commences from the point of red heat visible in day-light, and the whole range be divided into 240 equal parts, it will be found that Swedish copper melts at 28; gold at 32 ; iron at from 130 to 150 degrees : above this point, the cubes could not be heated. But, if one of these clay squares be put into an oven where other materials, such as bread, earthen-ware, etc. are to be baked, they may be usefully employed, for regulating the necessary degree of heat.