The same country, however, which bad Ow birth to the thermometer, began. its improvement. After the •principle of the barometer was establish ed, the members of the Academy del Cimento, founded at Florence in 1657, and supplied with Ube, tat funds by the Grand Duke of Tuscany, had, among other interesting physical researches, resumed the application of the thermometer ; and instead of air, they substituted alcohol ore spirit of wine, another very expansible fluid not affhcted by pressure, while they attached to the tube a scale graduated on a re gular plan, though directed by no very precise men. mires. The instrument so constructed, but some. what varied in its forie being copied by Italian art ists, was dispersed over Europe under the name of the Florence Glass. From its careless execution, however, in the hands of itinerant venders, this ther mometer, or rather theimonioscope, appears never to have obtained an established reputation.
The great object was to bring thermometers to an exact correspondence. It. was expedient, therefore, not only to select a proper ituid, but to adopt an uni form and consistent scale. Alcohol, linseed oil, and mercury, had been successively tried. The gradua tion was at first drawn from the temperature of col lam and deep caves, which, indicating the natural heat of the globe, had long been considered invari able; but more enlarged experience discovered the inaccuracy of that supposition, and showed the mean temperature to be material/3Y modified by the latitude of the place, and its elevation above the level of the sea. Congelation, or railer the inverted pro cess, the thawing of ice, or the melting of snow,. was then found to remain fixed ; a most important fact, which had been first noticed by .Giiricki, but overlooked till a considerable time afterwards. A stationary point was hence obtained, from which to commence the thermometer scale. But different modes were pursued for determining the divisions. Amontone, reverting to the air-thermometer in spite of its acknowledged defects, found that the elasticity of air compressed in the bulb, and able at the tem perature of melting snow to support a column of mercury fifty-four inches higle was capable of rais ing this to seventy-eight i • at the beat of boil ing water ; and he seemed contented in framing a rude standard, with merely dividing the intermediate space into inches and half-inches.
But about the same, or nearly at the beginning of the eighteenth century, Newton himself cast a keen though rapid glance on the subject of heat, and proposed a thermometer of a much simpler and more elegant construction. Having adopted linseed oil as a fixed and uniform substance, capable of great dilatation, he discovered by experimeat, that distinguishing the capacity of the bulb into ten thousand equal parts, the liquid expanded 256 parts, from melting snow to blood beat, and 725 Term to that of boiling water. These numbers, however, being inconveniently large, he reduced them somewhat more than twenty times, adopting 12 and 34 as the proportional divisions on his scale.
But oil,. being so viscid a substance, was found to and collect on the inside of the tube; and this therrannettor, constructed• es a right priss elpie, never came into general aso.
- Banner, the Danish astronomer wits made thee discovery of the-progressive motion of light, wile the float who proposed mercury as the fittest arid for then tnometers; anclifelley and Am:intone about the same time, that it expands uniformly with heat, and remains nearly stationary at the point of boiling water. On this principle, Delisle, of St Petersbargh, constructed', in t7SS, a mercurial thermometer, with a descending scale, the distance from freezing to boiling water occupying 158, or, in round numbers, 150 divisions, of which the bulb itself contains 10;000'. A more ingenious method, but perhaps too refined, for graduating thermometers, was proposed by Renaldini in 1034. It consisted in adopting the scale in the successive temperatures produced by mixtures in the different proportions of twelve parts of water at the moment of thawing and of ebullition. This suggestion led to a• very important inference, since it proved that mercury expands uniformly with equal additions of'heat, while alcohol swells constant ly in a rising progression. But the capital improve ment of the thermometer was effected by the skill and perseverance of Fahrenheit, whose name has remain• ed justly attached to the instrument. This ingenious person originally a merchant at Dantzic, who had the misfortune to fail in business, was induced, by his' taste for mechanics and chemistry, to have re course to the manufacture of thermometers, as the means of gaining a slender livelihood. But not meet . log with sufficient encouragement at home, he re moved, about the year 1720, to Holland, the great emporium of the arm, and- fixed his future residence at Amsterdam. He new preferred mercury to alco hol fbr filling his• thermometers; and, adopting the division of the bulb•into 10,000 parts, he reckoned 6+ of them as the aspersion between freezing to blbod-heat; and St as- the- contraction from the same point to what he considered as extreme cold, or that produced by the rnixtur• of salt with snow. These• numbers were extremely convenient, being found by a• repeated• bisection. With respect to the heat of boiling. water, Fahrenheit discovered the im portant-fact, that it varies with. the state of atmo spheric pressure. Taking the mean, however, he reckoned 150 degrees. from freezing- to ebullition, and, therefore; marked this pohit at t12 on his scale. The- thermometer owes its improvement to Celsius, proffissor at Upsal; who in 174Q- placed' the cons ntercement of the scale at cengelation, and divided the interval thence to. boiling water into an hun dred degrees, extending such• a portion. downward. as might be wanted: This centesimal' thermometer brazenly-the same- as•what the have•since called- the centigrade, which, from- its fitness- and shnplicity, deserver we be' universally adopted.