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 Santoria, who resided at Padua; and in this work he claims the honour of having invented the thermometer. Cornelius Drebble, of Alkmaar in Holland, makes the same claim, and after carefully examining the evidence, it appears, that although Santoria was the first to point out the use of this instrument, Drebble 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; and 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 of air within became contracted in bulk. A scale was then fixed alongside the tube, divided into certain degrees, so that the several changes could be measured as correctly as might be expected from the simplicity of the contrivance.

Such an invention was not long before it attracted notice; and after a few years the celebrated Robert Boyle, who had turned his attention to natural philosophy, and already made great improvements in the air-pump, devised an alteration in what might be called the form of the heat measurer. He left the tube open at both ends, and turned one of them upwards in a curve; this he sealed hermetically, by melting the glass to a vessel, on the top of which a hole was pierced, and the pressure of the atmosphere caused the spirit to rise and descend, in the upright portion of the tube, as the condition of the atmosphere was changed. Boyle, who was a son of the Earl of Cork, was a man distinguished in every way for noble qualities of mind and heart. After travelling through Europe, he settled in England; and during the great civil war which was waged between Charles I. and his Parliament, he had the good fortune to enjoy the favour of several eminent men on both sides; and having thus obtained protection both for person and property, he was enabled to follow his literary and scientific pursuits at leisure and in peace.

Boyle's chemical experiments date from the year 1646; and in all probability it was shortly after this period that he first turned his attention to the improvement of the thermometer. He settled at Oxford in the year 1654, and resided there till 1668, being during that time a member of the association which

was then termed "the Invisible College," and which afterwards obtained a permanent existence, and has obtained so much reputation under the title of 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 spirits 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; but 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 philosophers-and the Genevan professor, Saussure, especially-have employed it in the ascent of Mount Blanc and other lofty mountains in the Alpine district of Europe.

Horace Benedict de Saussure, whose father was also a philosopher, was, at the age of twenty-two, appointed to the chair of philosophy in the college of Geneva; and for five-andtwenty years he discharged the duties of a public teacher,taking advantage of the intervals between his official labours to ascertain the natural phenomena of the sublime and romantic district in which he was born. From his very childhood he had indulged this passion; and before he was eighteen years of age, he had explored the mountains in the neighbourhood of his native place. These excursions only created in him new desires for the indulgence of his curiosity; and he became more eager than ever to explore more closely the lofty heights of the Alpine mountains, on whose barren and exalted summits, and in whose dark and yawning ravines are written the records of the world's history, before man became an inhabitant of the earth, and where nature seems to reign supreme in solemn majesty. At length, in the year 1760, alone, and on foot, he made his way to the glaciers of Chamouni, then little visited even by those who lived in the locality, and which were almost altogether unknown to the world in general.

The ascent and descent were both difficult and dangerous, but it was accomplished in safety; and the next year, Saussure returned to renew his observations. From that time, the spirit of exploration was not to be restrained; and year by year he made not only excursions, but 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 different times by

eight different routes, and made sixteen other excursions to the centre of the mountain mass. He went over the Vosps 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 the other parts of France. And all this he did with his mineralogist's hammer in his hand, clambering up to every peak to observe the various strata, and making his notes on the very spot, where the different peculiarities existed, which he had set out to describe.

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, surrounded by the winds which howl among the heights and rush down like the blasts that sweep across the stormy ocean, he remained three hours and a half, noting the natural phenomena of that sublime district.