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.

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

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.

Up to the time of Sir Isaac Newton, mercury and spirit had been the only materials used for thermometers, but he was dissatisfied with them both, and adopted linseed-oil, a substance which has nearly the same power of expansibility, while it may at the same time be subjected to both very high and low degrees

of temperature, without either freezing or boiling. But an almost equal objection existed to the use of oil, for in time it became viscid, and adhered a good deal to the middle of the tube; a fault which prevented the observations being depended upon, and the use of it, consequently, in the construction of thermometers has of late years been entirely discontinued.

Mercury is now the only substance used for thermometers, and its first application has been variously ascribed to Dr. Halley, and Mr. Romer, the discoverer of the motion of light. According to Dr. Boerhaave, Romer invented the mercurial thermometer in 1709, but it was not till 1724 that any knowledge of it was obtained in this country, during which year an account of the thermometer invented by Mr. Fahrenheit, of Amsterdam, was first read to the Royal Society. In that paper 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, although that arranged by M. Reaumur is usually employed in France. The main difference between the two consists in the gradation of the scale-Reaumur fixing his zero at 32 degrees of Fahrenheit, and dividing the ranges between that point and the point of boiling water into 80 degrees, while Fahrenheit takes a scale of 212 degrees between his zero and the boiling point.

It is said that Fahrenheit obtained his zero by having mercury exposed in a tube to intense cold, in Iceland, during the year 1709. He then immersed the tube in freezing water, and found that the mercury stood at the 32nd degree above. On immersing it in boiling water, it stood at 212 degrees.