THERMOMETER is an instrument by which the temperatures of bodies are ascer tained. It consists of a glass tube with a capil lary bore containing in general alcohol or mer cury. When the liquid expands or contracts by variations in the temperature of the atmo sphere, or by the instrument being immersed in the liquid or gas which is to be examined, the state of the atmosphere, liquid, or gas, with respect to caloric, is indicated by a scale which is either applied to the tube or engraven on its exterior surface.
The end proposed by a thermometer is the measurement of the temperature of any body with relation to the temperature of some other substance, as of water at the point of freezing ; but the measure so obtained must not be understood to express the absolute quantity or density of caloric in any body, it being well khoWn that different substances, though exhibiting the same apparent temperature, contain very different quantities of caloric according to their capacities for that element.
The thermometer was in use in the begin ning of the 17th century, but it is not known precisely to whom the honour of the invention is due. A physician of Padua named Santorio, in his 'Commentaries on Avicenna' (1626), claims it for himself. It May however have happerred with this, as with other scientific discOveries, that the idea of the instrument occurred to two persons or more about the same time.
The earliest kind of thermometer consisted of a long glass tube containing air, which was hermetically sealed. at the upper end, while the lower was made to enter into a vessel of coloured spirit. By the pressure of the atmo sphere the spirit was made to occupy part of the tube ; and the variations in the tempera ture of the atmosphere causing the column of spirit to ascend or descend in the tube, allowed the degree of temperature to be measured by a scale applied to the latter. The defects inseparable from such thermometers are, that the dilatations of the air are not proportional to the increments of heat, and that the length of the column of spirit varies with the tempe rature of the atmosphere. Thus the indica. tions afforded by the thermometer are ren dered erroneous, or require corrections which it is difficult to apply.
About the middle of the 17th century the members of the Academia del Cimento caused thermometers to be constructed in which, instead of air, alcohol, or spirit of wine, was employed. Alcohol dilates and contracts
considerably with the variations of temperature to which it may be subject, though not in so great a degree as air. It is also capable of measuring very low temperatures but; as it is brought to a boiling state sooner than any other liquid, it cannot be employed to ascer tain a high degree of heat.
The idea of employing mercury for tho purpose of measuring degreeS of heat by its expansion is supposed to have first oocurred to Dr. Halley. Tho invention is ascribed to Romer, but it was not till 1721 that such a thermometer was known in this country. Tho advantages of mercury over alcohol and air, as a measure of temperature, are, that its expan sions are more nearly proportional to the increments of calorie than those Which' take place in either of the other fluids; it it easily deprived of air ; and its power to eondnct heat being considerable, the thangeS of its volume by changes of temperature in the surrounding medium take plate more rapidly than those of any other fluid adept the gases.
The scale which has been in general use ih this country since the year 1721, is supposed to have been 'invented by Fahrenheit. It is quite unknown on what ground he made choice of the fixed points on hie scale, dr the number of graddations between them ; but it is thought that one of the fixed points was that of boiling water, and that the other, which is the zero of the scale, was that at which the top of the column stood when the instrument was exposed to an intense cold in Ieeland, in 1709. The extent of the scale between this last point and that of boiling water is divided into 212 parts, and the point of trebling water is at the thirty-second division front the zero point. That which is called Reaumni's scale has the interval between the points of freezing and boiling water determined by experiment, and the distance between them is divided into eighty parts, the zero of the scale being at the freezing point. A third scale, called Centi grade,' has been much in use among the philosophers of the Continent within the last fifty years. It was invented by Celsius, Swede, and it differs from that of Iltaumnr only in the distance between the points of freezing and boiling water being divided into 100 parts. The length of each degree in this thermometer, as well as in that of Reaumur, is greater than in the scale of Fahrenheit.