THERMOMETER, an instrument for measuring the degree of heat or cold in any body. The first fbrm of this instru ment for measuring the degrees of • heat and cold, was the air thermometer. It is a well known tact that air expands with heat so as to occupy more space than it does when cold, anti that it is condensed by cold so as to occupy less,spacethan when warmed, and that this expansion and con densation is greater or less according to the degree of heat or cold applied. The principle then on which the air-thermome ter was constructed is very simple. The air was confined in a tube by means of some coloured liquor ; the liquor rose or fell according as the air became expand ed or condensed. What the first form of the tube was, cannot now perhaps be well known ; but the following description of the air-thermometer u ill fully explain its nature, it consists of a glass tube, B E, (Plate Miscel. XVI. fig. 4.) connected at one end with a large glass ball, A, and at the other end immersed in an open ves sel, or terminating in a ball, D E, with a narrow orifice at ; which vessel or ball contains any coloured liquor that will not easily freeze. Aquatimti.s tinged of a fine blue colour with a solution of vitriol or copper, or spirit of wine tinged with co chineal, will answer this purpose. But the ball, A, must be first moderately warmed, so that a part of the air contained in it may be expelled through the orifice, D; and then the liquor pressed by the weight of the atmosphere will enter the ball, D E, and rise, ihr example, to the middle of the tube, at C. at a mean temperature of the weather ; and in this state the liquor by its weight, and the air included in the ball, A, &c. by its elasticity, will counter balance the weight of the atmosphere. As the surrounding air becomes warmer, the air in the ball and upper part of the tube, expanding by heat, will drive the li quor mto the lower ball, and consequently its surface will descend; on the contrary, as the ambient air becomes colder, that in the ball is condensed, and the liquor, pressed by the weight of the atmosphere, will ascend ; so that the liquor in the tube will ascend or descend more or less ac cording to the state of the air contiguous to the instrument. 'l'o the tube is affixed
a scale of the same length, divided up wards and downwards from the middle, C, into 100 equal parts, by means of which the ascent and descent of the li quor in the tube, and consequently the variations in the cold or heat of the atmo sphere, may be, observed.
The air being found improper for mea suring with accuracy the variations of heat and cold, according to the form of the thermometer which was first adopted, another fluid was proposed about the middle of the seventeenth century by the Florentine Academy. This fluid was spirt of wine, or alcohol, as it is now gene rally named. The alcohol being colour ed, was inclosed in a very fine cylindri cal glass tube previously exhausted of its air, having a hollow ball at one end, A, (fig. 5.) and hermetically sealed at the other end, D. The ball and tube are filled with rectified spirit of wine to a convenient height, as to C, when the wea ther is of a mean temperature, which maybe done by inverting the tube into a vessel of stagnant coloured spirit, under a receiver of the air-pump, or in any other way. When the thermometer is proper ly filled, the end 1) is heated red hot by a lamp, and then hermetically sealed, leaving the included air of about one-third of its natural density, to prevent the air which is in the spieit from dividing it in its expansion. To the tube is applied a scale, divided from the middle, into 100 equal parts, upwards and downwards. As spirit of wine is capable of a very con siderable degree of rarefaction and con densation by heat and cold, when the heat of the atmosphere increases the spirit dilates, and consequently rises in thetube ; and when the heat decreases, the spirit descends, and the degree or quantity of the motion is shown by a scale.