[Scientific American. / Volume 5, Issue 50, p. 395 August 31, 1850]

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 rival claimants. There seems but two, however, whose titles are worthy of notice.

The Italian writers generally give the honor to their countryman, Santorio Santorio, long a physician at Venice, and afterwards a professor at Padua, and who had obtained just celebrity by his discovery of the insensible perspiration of the animal frame. The Dutch philosophers, on the other hand, unhesitatingly ascribe it to Cornelius Drebbel, a physician of Alkmaar, who appears to have enjoyed a high reputation as a chemist, a mathematician, and an inventive mechanical genius. Dr. Santorlo expressly claims the invention as his own, and he is supported by Borelli and Malpighi. The title of Drebbel is considered as undoubted by Boerhave and Musschenbroek. It would now, perhaps, be difficult 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 until 1626; and although thermometers are alluded to by Robert Fiud 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 Santoria appears to have reached the northwest 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 he justly entitled to the merit of a discoverer.

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

The thermometer ascribed to Santorio and Drebbel is precisely the same in form and principle; it consists of a glass tube, with a ball on one of its extremities, 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. As the ball cools the included air diminishes in volume, and the liquid is forced into the stem by the pressure of the atmosphere, until it replaces the volume of air which was expelled by the heat. When a heated body is applied to the ball, 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 heat, 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 called 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. In the obscure and often almost unintelligible writings of Dr. Robert Flud, published at the beginning of the seventeenth century, frequent mention is made of the thermometer or as he calls it speculum calendarium; and the common air is repeatedly figured in his singular work, De Philosophia Moysiaca, published in 1638, with its stem divided equally into an ascending and a descending series, each of seven degrees, respectively appropriated to summer and winter. 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.