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.

PIKE 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 ex'cellent 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

EVANGELISTA TORRICELLI,

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

purpose than water, and therefore used, in place of it, mercury, which is about fourteen times as heavy. Having closed a glass tube hermetically at one end, he filled it with mercury, and then brought the open end inverted into a vessel partly filled with the same substance, taking care that the end of the tube should be under the surface of the mercury in the open vessel. He thus observed that the column in the tube contracted till the top of it stood at between twenty-nine and thirty inches above the mercury in which it was immersed. Having marked the specific gravity of the mercury, the weight of the column of air between the mercury and the top of the tube became of course apparent, from the respective proportions of the columns of air and mercury and the whole length of the tube. It should be stated, that in 1631, that is, twelve years before Torricelli's observations, Descartes, the French philosopher, had made the same observation, although he does not appear to have turned it to any account.

This was the first and the great step; but whether Torricelli is entitled to the honour of having been the first to discover the true reason of the depression of the mercury, is uncertain; at any rate, there was at once an end of the "vacuum" assertions, and a great step was gained towards sound philosophical principles, and to that merit he is most decidedly entitled.

The subject had excited too much attention to be dropped ; and Pascal Mersenne in France, and Boyle in England, took it up. Of these, Pascal appears to have been the most sensible and rational observer. He very reasonably argued, that if it were the column of air which occasioned the alteration of the column of mercury, the higher the point in the atmosphere, the higher the mercury would stand in the tube; and Boyle had well prepared the way for him by testing the barometer with airs of different densities, by means of the air-pump.

To carry this principle to some practical conclusion, Pascal requested his friend, M. Perrier, to ascertain the height at

which the mercury stood at the base and on the summit of the Ruy de Dome, one of the loftiest mountains in the province of Auvergne. The result perfectly answered his expectations. At the base, the mercury stood at a height of 26 inches, while on the summit it was only 23 inches; the mountain being between three and four thousand feet above the level of the sea. A like result was afterwards ascertained by Pascal himself; and he also discovered that the same rule prevailed and was very sensibly shown, in the ascent of a private house and a church tower.

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Thus the fact was satisfactorily established, that the weight of a column of air was equal to that of a column of mercury about twenty-eight inches high, that is, a pressure of about fifteen pounds on a square inch.

The barometer only required the addition of an index and a weather-glass, to give a fair and true announcement of the state and weight of the atmosphere. The instruments are now manufactured in several different forms, but the principle is the same in all, and repeated observations during the ascent of the loftiest mountains in Europe and America, have confirmed the truth of barometrical announcements; for by its indications, the respective heights of the acclivities in high regions can now be ascertained by means of this instrument better than by any other course, with this advantage, too, that no proportionate height need be known to ascertain the altitude.

In navigation the barometer has become an important element of guidance, and a most interesting incident is recounted by Capt. Basil Hall, indicative of its value in the open sea. While cruising off the coast of South America, in the Medusa frigate, one day, when within the tropics, the commander of a brig in company was