An instrument for measuring the temperature of bodies; founded upon the principle of augmentation in volume of fluids, in proportion to their absorption of caloric; and as regards aeriform fluids, the principle is probably very correct: but solids, and still more liquids, expand unequally, by equal increments of heat. Thermometers were invented about the beginning of the seventeenth century; but a knowledge of their author is involved in some obscurity. For the first half century, after their introduction, they were made in a very rude and imperfect manner; but they were at length considerably improved by the Florentine academicians, and received subsequent ameliorations from Mr. Boyle, Dr. Halley, and Sir Isaac Newton, as well as from contemporaneous philosophers on the continent. The changes which the instrument underwent in their hands, (described in the Oxford Encyclopaedia,) we shall not here insert, as all that had at that time been proposed, were liable to many conveniences, and could not be considered as exact standards for pointing out the various degrees of temperature.

The thermometers which at present are in most general use, are Fahrenheit's, De l'lsle's, Reaumur's, and Celsius's. Fahrenheit's are used in Britain, De l'lsle's in Russia, Reaumur's, and the thermometer centrigade, in France, and Celsius's, the same as the last named, in Sweden. They are all mercurial thermometers.

Fahrenheit's thermometer consists of a slender cylindrical tube, and a 3mall longitudinal bulb. To the side of the tube a, is annexed a scale b, which Fahrenheit divided into 600 parts, beginning with that of the severe cold which he had observed inlceland in 1709, or that produced by surrounding the bulb c of the thermometer with a mixture of snow or beaten ice, and sal ammoniac, or sea salt. This he apprehended to be the greatest degree of cold; and accordingly he marked it, as the beginning of his scale, with 0; the point at which mercury begins to boil, he conceived to show the greatest degree of heat, and this he made the limit of his scale. The distance between these two points, he divided into 600 equal parts or degrees; and by trials, he found that, the mercury stood at thirty-two of these divisions, when water just begins to freeze, or snow or ice just begins to thaw; it was, therefore, called the degree of the freezing point. When the tube was immersed in boiling water, the mercury rose to 212, which, therefore, is the boiling point, and is just 180 degrees above the former, or freezing point.

But the present method of making the scale of these thermometers, which is the sort in most common use, is first to immerge the bulb of the thermometer in ice or snow just beginning to thaw, and mark the place where the mercury stands, with number 32; then immerge it in boiling water, and again mark the place where the mercury stands in the tube; which mark, with the number 212, exceeding the former by 180, dividing therefore the intermediate space into 180 equal parts, will give the scale of the thermometer, and which may afterwards be continued upwards or downwards at pleasure. Other thermometers of a similar construction have been accommodated to common use, having but a portion of the above scale. They have been made of a small size and portable form, and adapted with appendages to particular purposes; and the tube, with its annexed scale, has often been inclosed in another thicker glass tube, also hermetically sealed, to preserve the thermometer from injury.

In 1733, M. De l'Isle, of Petersburgh, constructed a mercurial thermometer, on the principles of Reaumur's spirit thermometer. In his thermometer, the whole bulk of quicksilver, when immerged in boiling water, is conceived to be divided into 100,000 parts; and, from this one fixed point, the various degrees of heat, either above or below it, are marked in these parts on the tube or scale, by the various expansion or contraction of the quicksilver, in all imaginable varieties of heat.

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 thermometer ought more properly to be called De Luc's thermometer, for it was first 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 physicians, 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 superseded 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 e 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 H,