THE ancients were unacquainted with any more certain mode of marking the variations of temperature, than the indications of the senses, and the limited knowledge derived from observing the melting or combustion of different substances. In modern times, instruments have been invented for noting variable degrees of heat and cold, which, under the designation of thermometers, or thermoscopes, pyrometers, or pyroscopes, are now in general use in every part of the civilized world. Their names are derived from the Greek terms us, ug, signifying heat, fire, and μirgov, oxoxos, a measure, an investigator.

The principle on which all such instruments are constructed, is the change

of bulk which every body undergoes by alteration of its temperature.

All homogeneous bodies, except water, within a few degrees of its freezing point, expand by heat and contract by cold.* Their expansion, then, may afford a relative measure of the increase of temperature; and their contraction, of its diminution. This law holds good in gases, liquids, and solids; and, accordingly, matter in those three states of existence has been employed in the construction of instruments for measuring the intensity of

heat and cold.

were

The changes of volume which gases or acriform bodies undergo, were first employed for this purpose; liquids, such as spirit of wine, oils, or mercury next used; and lastly, the changes in the bulk of solids were applied to measure the variations of higher temperatures, which would have too much expanded gaseous and liquid bodies.

The designation of thermoscope or pyroscope might be, with most propriety, applied to such instruments; but, in conformity to common usage, it is proposed in this treatise to apply the

Clay, a seeming exception, is not a homogeneous

substance, of which afterwards.

Of the Common Thermometer. §1. History and Construction of the Thermometer.

THE invention of the thermometer, like almost every other discovery of great utility, has been claimed for different philosophers; and national vanity has occasionally been enlisted in support of the pretensions of rival claimants. There seem, however, but two whose titles are worthy of notice.

The Italian writers generally give the honour to their countryman Santorio Santorio, long a physician at Venice, and afterwards a professor at Padua, who flourished about the beginning of the seventeenth century; and who had obtained just celebrity by his discovery of the insensible perspiration of the animal frame: the Dutch philosophers as unhesitatingly ascribe it to Cornelius Drebbel, a physician of Alkmaar, who appears to have enjoyed a high reputa

tion as a chemist, a mathematician, and an inventive mechanical genius.

B

Santorio expressly claims the invention as his own, and he is supported by Borelli and Malpighi; the title of Drebbel is considered as undoubted by Boerhaave § and Musschenbroek.|| It would now be difficult, perhaps, to decide the controversy; but it is worthy of remark, that Santorio, who was born in 1561, and died in 1636, did not publish his claim to the invention till 1626;** and, although thermometers are alluded to by Robert Flud, within the first quarter of that century, yet as he travelled both in Germany and Italy for six years, we can draw no inference from that circumstance. Certain it is, that thermometers were constructed about the same time, both in Italy, and in Holland, on the same principle; and though the instruments of Drebbel were well known in Holland and England, before the fame of Santorio appears to have reached the North-West of Europe, the most recent writers have generally considered the latter as the real inventor of the thermometer. It is, however, by no means improbable that each may be justly entitled to the merit of a dis

coverer.

Be this as it may, the instrument was, from its imperfect construction, of little use in the hands of either, and required the successive labours of different philosophers to render it a tolerably accurate indicator of the variations of temperature.

The thermometer ascribed to San

torio and to Drebbel, is pre- Fig. 1. cisely the same in form and principle. It consists of a glass tube, with a ball blown on one of its extremities A, (fig. 1,) and having the other end open. A portion of the air in the ball is expelled by heat, and then the open end of the tube is immersed in any liquid contained in the cup c. As the ball cools, the included air diminishes in volume, and the liquid is forced into the stem, as at b, by the pressure of c the atmosphere, until it re

places the volume of air which was

expelled by the heat. When a heated body is applied to the ball A, the air will again be expanded, and depress the liquid in the stem; and, if this stem be a cylinder, a scale of equal parts applied to it will enable the observer to form some idea of the difference between the relative temperature of bodies applied to the ball. On the removal of the heated body, the volume of the included air again diminishes, and the liquid again rises in the stem by atmospheric pressure, until the elasticity of the air within the instrument is in equilibrio with that of the surrounding atmosphere. Instruments constructed on this principle are termed air thermometers; because their action depends on the elasticity of air; and from their having been originally employed to mark the changes of atmospheric temperature, they are described by the older writers under the name of weather-glasses; a denomination also given to barometers.

Drebbel appears to have devised a variety of the instrument more delicate in its indications. The globular form of the common bulb, and its small size, rendered it less susceptible of slight changes than a flattened bulb of larger diameter; and Boerhaave describes the bulb of Drebbei's thermometer, as composed of two shallow segments of large spheres, as in fig. 2. A, united at their edges, and in fig. 2. B, where it is seen in profile.

Fig. 2. A.

Fig. 2. B.

In the obscure, and often almost unintelligible, writings of our countryman, Dr. Robert Flud, published about the beginning of the seventeenth century, frequent mention is made of the thermometer, or, as he calls it, speculum

Calendarium; and the common air thermometer is repeatedly figured in his singular work, De Philosophia Moysiaca, with its stem equally divided into an ascending and descending series, each of 7 degrees, respectively appropriated to winter and to summer. It is obvious, that the size of an air thermometer, on such principles, is only limited by convenience, and the length of the column of liquid which the pressure of the atmosphere can sustain in the tube. As originally made, they were unwieldy, they could not be applied to high temperatures, and were, besides, liable to two very important objections, as indicators of the atmospheric changes of temperature, they were liable to be affected not only by heat and cold, but by the varying pressure of the atmosphere; and the scales adapted to them were arbitrary, and without fixed points for the comparison of observations made with different instruments.

The first objection was foreseen and obviated by the scientific members of the Florentine academy del Cimento, assembled under the auspices and patronage of Fernando II., Grand Duke of Tuscany. In the first article in the published transactions of that learned body, we find a full description and delineation of a thermometer from which the influence of atmospheric pressure is excluded. The expansion of spirit of wine is employed to ascertain the temperature, instead of the dilatation of air; and the instrument is sealed hermetically, as it is termed, or has its orifice closed by melting the glass, after the introduction of as much spirit as fills the bulb and a portion of the stem. The method employed by the Florentine academicians is nearly that still used by the makers of the instrument; namely, by heating the bulb in the flame of a lamp, to expel the air, and then immersing the open end of the tube in the liquid destined to fill the thermometer. As the ball cools, the atmospheric pressure forces the liquid into the stem and ball, to supply the vacuum; and the orifice is closed by melting with the blowpipe the end of the tube, from which any excess of the liquid may be previously expelled by again heating the ball. (Fig. 3.)

The Florentine academicians appear also to have been aware of the neces

Folio, Goyd, 1638.

+ Saggi di Naturali Esperienze.

sity of adapting some fixed Fig. 3. scale to the tube; but their attempts were not very successful. They described the thermometer as consisting of a ball and tube of such relative size, "that on filling it to a certain mark of its neck with spirit, the cold of snow and ice will not cause it to fall below 20 degrees measured on the stem; nor, on the other hand, the greatest heat of summer expand it more than 80 degrees."* This method is undoubtedly erroneous, inasmuch as the last point could be of no determinate temperature; and their method of graduation is in itself rather rude. The tube is directed to be divided by compasses into ten equal parts, these divisions are to be marked "by a little button of white enamel; and these may be further subdivided by the eye, and the intermediate degrees marked by buttons of glass, or of black enamel."'

This instrument was variously modified by them to suit different purposes. The ball was occasionally enlarged, and the tube reduced in thickness to render the instrument more sensible; and in the work already quoted, we find a figure of a thermometer of this sort, with the stem spirally twisted to render it more portable, and less liable to accident.

Another invention of those philosophers to indicate changes of temperature may be here noticed. It consisted of hermetically sealed spherules of glass, of different specific gravities, introduced into a wide tube filled with pure spirit. The degree of the Florentine thermometer at which each sank was noted, and by hanging this instrument in an apartment, it somewhat slowly showed the variations of the temperature of the surrounding air. Imperfect as these attempts were, they paved the way to very important improvements in thermometers.

The indefatigable Boyle appears early to have turned his attention to the improvement of the thermometer, and his first attempts were on the air thermometer, or the weather-glass, as it was then styled. He rendered the instru

Saggi di Naturali Esperienze, p. 4 + Saggi, p. 10.