THERMOMETER; from the Greek words beglzos, heat, and PPrg", a measure ; an instrument adapted for the comparative estimation of the temperature of bodies.
That the idea of any instrumental measurement of heat is wholly of modern invention seems beyond a doubt, since in the writings of the ancients we find no trace of any such comparative scale except the rude indications of our natural feelings. Since we are absolutely ignorant of the state of substances deprived of, or saturated with heat; since our senses, though adapted to the perception of it, are inca pable of forming any accurate measure of its effect, or of computing its real quantity upon any inde pendent standard as we can do the pressure of the atmosphere by knowing the weight of an equipon derant column of mercury, we are obliged to have recourse to a relative, not an absolute scale, and establish an arbitrary measure of heat by the ex tent of one of its acknowledged effects, that of ex pansion. (See the Article HEAT.) Hence, it is obvious, that in order to conceive the thermometer to be a true measure of heat, we assume that by equal increments of that agency, the expanding substance employed is uniformly en larged in bulk, a postulate not of easy proof, but which it has been ascertained in most cases of im portance is nearly true. But this we shall explain more fully afterwards; in the meantime we propose to give some account of the invention and history of the simple thermometer, with the various im provements which it has gradually undergone, and after some directions for its practical construction, proceed to the various ingenious modifications which it has received for different scientific pur poses, and we propose to conclude with some con siderations on the application of the thermometer to use, and the defects to which it is liable, without the most scrupulous attention.
Like most other important discoveries, that of the thermometer has had several claimants, the res pective merits of whom it seems nearly impossible at the present day definitely to determine. Though by some Galileo has been considered as the true inventor, and by others, Father Paul Sarpi, the Ve netian, this honour chiefly rests between Sanctorius, an Italian physician, and Drebbel, a Dutchman, both men of ingenuity and original minds. Many moderns support each of these claims, but Sancto rius (or Santorio) has, on the whole, most in his favour, since, as Martine justly observes,* no one but he demanded the honour of an inventor during his lifetime. His Commentaries on dvieenna, pub lished in 1626 are very interesting, since they con tain descriptions and curious wood cuts of the va rious forms of the thermometer which he proposed for medical purposes; and the extent to which he carried it. in other matters, is shown by his attempt described and figured at folio 22 of that work, to compare the heat of the sun's and moon's rays, an attempt which afterwards became one of the most delicate in natural philosophy.
Perhaps Sanctorius and Drebbel invented much about the same time this instrument, a circum stance by no means improbable, considering the then advancing state of physical science. Be this as it may, the instrument contrived by each was precisely similar, in which air was employed as the expanding substance, which in several respects is remarkably well fitted for this application.
The air thermometer, as shown at Plate DXXIV. Fig. 1, consisted of a ball of glass A with a stem B nearly filled with air, but having the lower part of the tube occupied by a coloured liquor, which, when the air in A was expanded, was forced into the recipient C, and hence a scale being applied to the stem, the dilatation of the enclosed air by heat was marked by the descent of the coloured fluid. In this form the instrument was obviously unfit for trying with any ease the temperature of fluids.
Boyle therefore subsequently proposed to include both the air and the coloured fluid in one bulb as shown at Fig. 2, where the tube AB which is open at top, reaches below the surface of the fluid nearly to the bottom of the receptacle C, into the neck of which the tube is hermetically sealed. The same philosopher, however, demonstrated the grand de fect of the air thermometer, that by a change of pressure in the atmosphere, as shown by the barometer, the elasticity of the inclosed air is altered independently of temperature, so as to render the indications of the same instrument not comparable at different times. The air thermometer was subsequently modified by the ingenious Hooke, in order to act as a barometer, which it obviously does, if the effects of tempera ture be corrected and those of pressure alone shown, just as on the other hand, if the result of variable pressure were neutralized, that of tempera ture would be truly expressed. Ilooke attached a mercurial thermometer to the original instrument, the temperature of which thus indicated gave the data for separating the influence of dilatation caused by heat, which was performed by means of a sliding scale. By this elegant modification Hooke converted the air thermometer into a marine ba rometer, which, however, was soon abandoned from the absorption that was found to take place of the excluded air by the coloured fluid. This defect has more lately been in a great measure remedied by the substitution of hydrogen gas instead of the common included air, by Mr. Adie, whu has reviv ed this instrument under a very elegant and porta ble form, and under the name of the sympiesometer. See MEmonoLoov.