In the following year, accompanied by his eldest son, hé 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.

This scale he obtained by ascertaining the capacity of the bulb, and dividing it into ten thousand parts, he found that the expansion of the mercury was just equal to two hundred and twelve of these parts when it was exposed to boiling water.

The thermometer constructed by Reaumur was a spirit thermometer. He divided the capacity of the ball into one thousand parts, and then marked off the divisions, two of which were equal to one of those parts. He found his zero by exposing the instrument to freezing water; and then plunging it into boiling water, he observed whether the spirit rose to exactly eighty of those divisions, and if it did not he strengthened or diluted the spirit until it rose. But this could give no fair indications of heat, as spirit boils long before it reaches the point of boiling water, and the one now termed Reaumur's thermometer is an improvement upon the instrument constructed by him.

Other kinds of thermometers have been invented for combined purposes. One of the chief of these is rather a barometer and thermometer united in the same instrument. Another, in which coloured sulphuric acid is employed as an indicator, is in fact two thermometers, each having a rectangular addition at the bottom, where the ends are joined and hermetically sealed. There are balls at the upper end of each of the upright tubes, and just as the air contained in each of the balls varies from the other, the spirit rises in the tube in which the air is most rarified.

Such are the several gradations through which one of the most important instruments in the service of the useful arts has been brought to its present state of perfection-one which has rendered invaluable aid in those more abstruse scientific investigations which have resulted in so much benefit to mankind.

THE BAROMETER.

IKE all great discoveries, the Barometer was found out by accident. The Duke of Florence had employed some pump-makers upon his premises, and they found that they could not raise the water above thirty feet, when the air in the tube was exhausted. In their dilemma they applied to the celebrated philosopher Galileo. He replied that nature had no power to destroy a vacuum beyond thirty-two feet; for, learned as Galileo was, he understood not the equipoising weight of atmosphere. It was left to his pupil Torricelli to make this discovery.

Evangelista Torricelli, who in early life distinguished himself for his mathematical and philosophical knowledge, was a native of Piancondoli, in Romagna, where he was born in the year 1608. By the care of an uncle, he received an excellent education at the Jesuit School in Faenza, where he became remarkable for his mathematical and scientific attainments. At twenty years of age his uncle sent him to Rome, and he there became intimate with Castelli, then

mathematical professor of the college of that city. About this

time Galileo was endeavouring to overturn the received doctrine that substances descended in speed according to their natural gravity; and that consequently, if two weights were to descend from a high position, the one which was ten times the weight of the other would reach the ground ten times as soon. Galileo had discovered the pressure of the atmosphere, and was convinced of the principle of its specific gravity, and of the opposition which it occasioned to the effect of the earth's attraction. He went, attended by several officials, to test its validity, and two stones, of very unequal weight, were dropped from the falling tower in Pisa. The truth was evident from the fact that the stones reached the ground nearly at the same moment; but it was in vain that Galileo pointed out that the difference in the time of their descent was entirely owing to the unequal resistance of the air. Prejudice had darkened reason too much for conviction to enter into the minds of the officials by whom he was accompanied.

These several experiments, and similar facts which had been educed by them, were too important to be overlooked by the acute mind of Torricelli; and he published two tracts,—one on the motion of fluids, and the other on mechanics,—which soon obtained the favourable notice of the venerable Galileo, by whom he was invited to Florence. After Galileo's death, which shortly took place, the Duke of Florence gave Torricelli the chair of mathematics in the Academy; and he thus became his friend's successor when he was about thirty-nine years of age.

As has been observed, Galileo had ascertained, through the representations of the workmen of the Duke of Florence, that water cannot be raised higher than thirty-two feet in a cylinder when the air is exhausted. With this circumstance Torricelli had also become acquainted; and being desirous of confirming the fact, or of discovering that the assertion was erroneous, he employed a more convenient medium for the