The thermometer at present used in France is called Reaumur's, but it is very different from the one originally invented by Reaumur in 1730, in which spirits of wine was used to indicate the degrees of expansion. The thermometer now in use in France is filled with mercury ; and the boiling-water, which is at 80, corresponds with the 212th degree of Fahrenheit. The scale, indeed, commences at the freezing point, as the old one did. The new thermo meter ought more properly tobe called De Luc's thermometer, for it wasfirst made by De Luc. When De Luc had finished the scale, and completed an account of it, he showed the manuscript to M. De la Condamine. Condamine advised him to change the number 80; remarking, that such was the inattention of physi cians, that they would probably confound it with Reaumur's. De Luc's modesty, as well as a predilection for the number 80, founded, as he thought, on philosophical reasons, made him decline following this advice ; but he found by experience, that the prediction of Condamine was too well founded. The thermometer of Celsius, which is used in Sweden, has a scale of 100 degrees from the freezing to the boiling-water point.
The thermometer centigrade, now used in France, has the scale divided in the same way. Many other thermometers have been used besides these, and consequently observations taken by them ; but it is unnecessary to describe any of these more minutely, as they are no longer used. Those who wish to read a more particular account of them may consult Dr. Martine's Essays. It must be admitted that disadvantages attend the adoption of the scales of each of the thermometers we have described, but hitherto the sanction of long usage in the countries where they have been introduced, has prevented their being super seded by any other.
A self-registering thermometer has been invented by Mr. Keith, of Ravel stone, which is considered as most ingenious and simple. a b, in the annexed figure, is a thin glass tube, about fourteen inches long, and three-fourths of an inch calibre, close or hermetically sealed at top. To the lower end, which is open, there is joined the crooked glass tube b e, seven inches long, and four tenths of an inch calibre, and open at top. The tube a b is filled with the strongest spirit of wine, and the tube b s with mercury. This is properly a spirit-of-wine thermometer, and the mercury is used merely to support a piece of ivory, or glass, to which is affixed a wire for raising one index, or depressing another, according as the mercury rises or falls. E is a small conical piece of ivory or glass, of such a weight as to float on the surface of the mercury. To the float is joined a wire, called the float-wire, which reaches upwards to li, where it terminates in a knee beft..ct right angles. The float-wire, by means of an eye at h, moves easily along the small harpsichord wire g h. 11 are two
indexes made of thin black oiled silk, which slideupwards or down wards with a force of not more than two rains. The one placed above the knee points out the greatest rise, and the one placed below it points out the greatest fall, of the thermometer. When the instrument is to be prepared for an observation, both indexes are to be brought close to the knee h. It is evident, that when the mercury rises, the float and float-wire, which can be moved with the smallest force, will be pushed upwards till the mercury becomes stationary. As the knee of the float-wire moves upwards, it will carry along with it the upper index I. When the mercury again subsides, it leaves the index at the highest point at which it was raised, for it will not descend by its own weight : as the mercury falls, the float-wire does the same; it therefore brings along with it the lower index 1, and continues to depress it tilt it again becomes stationary, or ascends in the tribe; in which case it leaves the lower index behind it, u it had formerly left the upper.
The scale to which the Indexes point is placed parallel to the slender harpsichord wire. That the scale and indexes may not be injured by the wind and rain,* cylindrical glass cover, close at top, and made so as exactly to fit the part g f, is placed over it.
As a knowledge of the correspondence between the thermo meters of Fahrenheit, Reaumur, and Celsius, are indispensable to the comprehension of the scientific labours of the French and German philosophers and authors, whether in the original languages, or the English translations, we have inserted a table in which the degree of any given temperature under 212• of Fahrenheit is expressed by those of Reaumur and Celsius: we omit De Lisle's, its use being confined to Russia. As, however, higher degrees of temperature may be required than those given in the table, the following rules are given for changing the degrees of any one of the scales into equivalent degrees of another ; viz. each degree of Fahrenheit is equal to four-ninths of one of Reaumur ; as Reaumur, however, reckons his degrees from the freezing point, and Fahrenheit 320 below this point, we must, when the number of Fahrenheit's degrees to be reduced' indicate a temperature above the freezing-point, first deduct 32, then multiply the remainder by 4, and divide the product by 9. The quotient is the corresponding number of degree. on Reaumur's scale. If the temperature indicated was less than the freezing point, we must also be careful to take the actual number of degrees, reckoning from the freezing point. Thus 4 degrees above Fahrenheit's zero is 28 below his freezing point • and this is the number to be reduced to Reaumur's scale.