THE THERMOMETER.

HE origin of the Thermometer, like that of the Mariner's Compass, remains in obscurity. We only know that the idea of measuring the degree of heat, which the atmosphere at different periods presents, was first conceived in Italy, that country which, during the latter portion of the Middle Ages, was distinguished by the attainments and discoveries of its scientific men.

In the year 1626, there was a book published entitled, Commentaries on the Works of Avicenna, by a physician, named Santorio, who resided at Padua ; and in this work he claims the honour of having invented the Thermometer. Cornelius Drebbel, of Alkmaar, in Holland, makes the same claim; and after carefully examining the evidence, it appears, that although Santorio was the first to point out the use of the instrument, Drebbel had also discovered and made its properties known before he heard anything of the invention of the Italian physician.

For some time after the invention of the Thermometer, it was chiefly used for ascertaining the changes of temperature alone, and the instrument was of the simplest description. A glass tube was formed with a ball at one end; the other end was open, and inserted in a vessel partly filled with mercury or coloured spirit-generally the latter. Previous to this, the air inside the instrument was heated by a lamp, so that when the temperature of the atmosphere was increased, it caused the air within the ball and the tube to be rarified. As this expanded, and occupied more space, it pressed down the spirit; on the contrary, when the temperature was reduced, its pressure upon the surface of the spirit decreased, and the latter was forced higher up the tube, as the quantity within became contracted in bulk. A scale was then fixed beside the tube divided into

degrees, so that the several changes could be measured as correctly as might be expected from the simplicity of the contrivance.

The invention soon attracted the attention of the celebrated Robert Boyle, who had already made great improvements in the Air-pump, and devised an alteration in the form of the heat-measurer. He left the tube open at both ends; the lower end was immersed in a small glass vessel containing both air and coloured spirit, and the vessel being formed with a neck, which closely encircled the tube, it was hermetically sealed to the latter. The variations in the temperature of the atmosphere caused the air in the vessel to expand or contract, and thus to press with more or less force on the surface of the spirit; the latter being consequently made to ascend or descend in the tube. Boyle, who was a son of the Earl of Cork, was a man distinguished for noble qualities of mind and heart. His chemical experiments date from the year 1646; shortly after which he turned his attention to the improvement of the Thermometer. He was one of the members of "The Invisible College," which was incorporated with the Royal Society.

In 1702, Amontons, a French philosopher, invented an Air Thermometer, which was about four feet long. It consisted of a tube open at both ends, one end turning up, and terminating in a ball with an aperture, so that there was the pressure of two atmospheres on an enclosed column of mercury, which was about twenty-six inches and a half in length. Some spirit, or other similar substance, floated on the top of the mercury; and in this a piece of wire was inserted, while on the top there was an index, which showed the various changes on the scale that was attached to it.

Some of these Thermometers were tolerably correct in their working; but they were all defective in one particular, inasmuch as the several expanses of the air are not exactly in proportion to the heat contained in the atmosphere; to remedy this, towards the middle of the seventeenth century the members of an Italian Academy had instruments constructed in which alcohol or spirit of wine was used instead of mercury. In that case the instrument was much like those of the present day. There was a tube with a ball at the bottom of it; and from this ball the air was expelled by heat, and mercury was introduced. The top of the tube was then hermetically

sealed; and as the degree of warmth without expanded or contracted the air, the spirit was either raised or depressed in the tube. Alcohol is very sensitive of the influence of heat, and expands very readily under its influence; but it has never been known to be frozen, and these spirit thermometers are therefore well adapted for ascertaining degrees of intense cold; though that very quality prevents it from being a good thermometrical medium for measuring high temperatures, as it boils at 176 degrees of Fahrenheit's scale, or 36 degrees below the point at which ebullition takes place in mercury. It has accordingly been frequently used to ascertain the degree of cold in elevated places; and several of the French philoso

phers-and the Genevan professor, Saussure, especially-have employed it in the ascent of Mont Blanc, and other lofty mountains in the Alpine district of Europe.

Horace Benedict de Saussure was, at the age of twenty-one, appointed to the Chair of Philosophy in the College of Geneva; and for five-and-twenty years he discharged the duties of a public teacher. In the intervals of his official labours, he loved to make excursions in the sublime and romantic country in which he was born; and before he was eighteen years of

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age he had explored the mountains in the neighbourhood. These excursions created in him new desires to explore more closely the lofty heights of the Alpine mountains; and in the year 1760, alone, and on foot, he made his way to the Glaciers of Chamouni, then little visited by those who lived in the locality. The ascent and descent were both difficult and dangerous, but they were accomplished by him in safety; and from this time Saussure, year by year, undertook many journeys to carry on his observations among the mountains in different parts of Europe. Between the years 1758 and 1779, he traversed the whole chain of the Alps no less than fourteen times by eight different routes, and made sixteen other excursions to the centre of the mountain mass. He went over the Vosges and the Jura, traversed the passes of Switzerland, trod the craggy heights of Germany; surveyed those of England, of Italy, and of Sicily and the adjacent islands; inspected the ancient volcanoes of Auvergne, and visited the mountains of Dauphiné and other parts of France. And all this he did with his mineralogist's hammer in his hand, clambering up to every peak that promised anything of interest, and making his notes on the very spot, where the different peculiarities existed which he had set out to describe; besides collecting specimens of the minerals and mountains.

In 1787, when forty-seven years of age, he ascended to the top of Mont Blanc, and in the intense cold of that lofty region he remained three hours and a half, noting the natural phenomena of that sublime district.

In the following year, accompanied by his eldest son, he encamped on the Col du Géant, at a height of 11,170 feet above the level of the sea, and remained there seventeen days without quitting his position. In the year after, he reached the summit of Monte Rosa in the Penine Alps, the last ascent of importance which he performed.

During his several journeys, while Saussure naturally turned his attention to the meteorological phenomena, he invented several philosophical instruments, the necessity for which he learned from his personal experience. Among others, a Thermometer for ascertaining the temperature of water at great depths; an hygrometer to show the quantity of watery vapour in the atmosphere; and an electrometer to develop its electrical condition.

The Thermometer which is now in general use is a slender

tube of glass, terminating in a ball containing mercury, the air having been expelled, and the tube afterwards hermetically sealed. The idea of employing mercury for the purpose of measuring degrees of heat by its expansion, is supposed to have first occurred to Dr. Halley; but he did not employ it, owing to the range of its expansion being much less than that of alcohol. Boerhaave ascribes the invention of the mercurial Thermometer to Römer in 1709; but it was not till the year 1724 that such a Thermometer was known in this country. In that year, a mercurial Thermometer which had been invented by Fahrenheit, of Amsterdam, in 1720, was described in a paper read to the Royal Society; in which it was shown that the

mercury more nearly represents the alteration in the amount of heat in the atmosphere, than either alcohol or air. Being easily deprived of the air it contains, and from its metallic quality, and ability to conduct heat rapidly, the change in its volume both quickly and accurately represents the alterations in the atmosphere.

Fahrenheit's thermometer is the one now in general use in this country. Römer (Réaumur) is used by the German authors; the French adopt that of Celsius, a Swedish philo