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THERMOMETER. 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 : bat 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 im proved by the Florentine academicians, and received subsequent ameliorations from Mr. Boyle, Dr. Halley, and Sir Isaac Newton, as well as from contempo raneous philosophers on the continent. The changes which the instrument underwent in their hands, (described in the Oxford Encyclopedia) we shall not here insert, as all that had at that time been proposed, were liable to many eonveniences, 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 lisle's, Reaumur's, and Celsius's. Fahrenheit's are used in Britain, De lisle'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 small 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 bad 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 thet degree of heat, and this he made the limit of his scale. The distance 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 begins to freeze, or snow or ice just begins to thaw; it was, therefore, 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 be sort in moat 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 meter 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.

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