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takes place in a man, when he acquires an idea, or a virtue, or a faculty; in a word, when he develops himself individually, what is the desire that instantly takes possession of him? It is the desire of publishing his sentiment to the world, of externally realizing his idea. As soon as a man acquires anything, as soon as his existence takes a new form in his eyes, assumes a higher value, to this new form, to this higher value, is immediately attached the idea of a mission; he feels himself obliged and impelled by his instinct, by an inward voice, to extend and establish in the world the alteration, the amelioration which has taken place in himself. We are indebted to this cause alone for our great reformers; the great men who have changed the face of the world, after being themselves changed, have been incited and governed by no other desire than this. So much for the change which operates upon the internal condition of man; let us take the other. A revolution is accomplished in the condition of society; it is better regulated, rights and property are divided more equally among individuals; that is to say the outward appearance of the world is purer and more refined, the actions, whether of government, or of men among themselves, are juster and better. Well! do you suppose that this outward appearance, this amelioration of external things, would not react upon the interior of man, upon humanity? All that has been said of the authority of examples, of customs, of exalted models, is founded on nothing else, if not upon this conviction that an external fact, good, reasonable, well-regulated, brings sooner or later, more or less completely, an external fact of the same nature, of the same merit; that a better ruled world, a juster world, makes man himself more just; that the interior is formed by the exterior, as the exterior by the interior; that the two elements of civilization are closely united one with the other; that centuries, that obstacles of all kinds may interpose between them; that it is possible they may submit to a thousand transformations before they are again united, but that sooner or later they will unite; that it is the law of their nature, the general fact of history, the instinctive belief of mankind.

20.-THE BAROMETER.

ARNOTT. [We have the permission of our friend, Dr. Amott, to extract for our · Half-Hours' his account of the Barometer. The work from which this is transcribed is entitled • Elements of Physics, or Natural Philosophy, General and Medical, explained independently of Technical Mathematics. Of this book the first volume was published some twenty years ago, and has passed through several editions. A portion only of the second volume has appeared. When we consider that this excellent book can only be completed at the rare intervals of leisure in a most arduous professional life,—that at the moments when the physician is not removing or mitigating the sufferings of individuals, he is labouring for the great benefit of all by such noble inventions as the Hydrostatic Bed,-we can only hope that the well-earned repose which wise men look to in the evening of their day, will give opportunity for perfecting one of the books best calculated to advance the education of the people that the world has seen. When Dr. Arnott has put the last labour to his · Elements of Physics,' it will remain for him to add one more claim to our gratitude by making it cheap.]

Galileo had found that water would rise under the piston of a pump to a height only of about thirty-four feet. His pupil Torricelli, conceiving the happy thought, that the weight of the atmosphere might be the cause of the ascent, concluded that mercury, which is about thirteen times heavier than water, should only rise under the same influence to a thirteenth of the elevation :-he tried and found that this was so, and the mercurial barometer was invented. To afford further evidence that the weight of the atmosphere was the cause of the phenomenon, he afterwards carried the tube of mercury to the tops of buildings and of mountains, and found that it fell always in exact proportion to the portion of the atmosphere left below it;-and he found that water pumps in different situations varied as to sucking power, according to the same law.

It was soon afterwards discovered, by careful observation of the mer curial barometer, that even when remaining in the same place, it did not always stand at the same elevation; in other words, that the weight of atmosphere over any particular part of the earth was constantly fluctuating; a truth which, without the barometer, could never have been suspected. The observation of the instrument being carried still farther, it was found, that in serene dry weather the mercury generally stood high, and that before and during storms and rain it fell :the instrument therefore might serve as a prophet of the weather, becoming a precious monitor to the husbandman or the sailor.

The reasons why the barometer falls before wind and rain will be better understood a few pages hence; but we may remark here, that when water which has been suspended in the atmosphere, and has formed a part of it, separates as rain, the weight and bulk of the mass are diminished : and that wind must occur when a sudden condensation of aeriform matter, in any situation, disturbs the equilibrium of the air, for the air around will rush towards the situation of diminished pressure.

To the husbandman the barometer is of considerable use, by aiding and correcting the prognostics of the weather which he draws from local signs familiar to him; but its great use as a weather-glass seems to be to the mariner, who roams over the whole ocean, and is often under skies and climates altogether new to him. The watchful captain of the present day, trusting to this extraordinary monitor, is frequently enabled to take in sail and to make ready for the storm, where, in former times, the dreadful visitation would have fallen upon him unprepared.—The marine barometer has not yet been in general use for many years, and the author was one of a numerous crew who probably owed their preservation to its almost miraculous warning. It was in a southern latitude. The sun had just set with placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch was proceeding, when the captain's order came to prepare with all haste for a storm. The barometer had begun to fall with appalling rapidity. As yet, the oldest sailors had not perceived even a threatening in the sky, and were surprised at the extent and hurry of the preparations; but the required measures were not completed, when a more awful hurricane burst upon them than the most experienced had ever braved. Nothing could withstand it; the sails already furled and closely bound to the yards, were riven away in tatters : even the bare yards and masts were in great part disabled; and at one time the whole rigging had nearly fallen by the board. Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and of the incessant peals of thunder, that no hu

man voice could be heard, and amidst the general consternation, even the trumpet sounded in vain. In that awful night, but for the little tube of mercury which had given warning, neither the strength of the noble ship, nor the skill and energies of the commander, could have saved one man to tell the tale. On the following morning the wind was again at rest, but the ship lay upon the yet heaving waves, an unsightly wreck.

The marine barometer differs from that used on shore, in having its tube contracted in one place to a very narrow bore, so as to prevent that sudden rising and falling of the mercury, which every motion of the ship would else occasion.

Civilized Europe is now familiar with the barometer and its uses, and therefore, that Europeans may conceive the first feelings connected with it, they almost require to witness the astonishment or incredulity with which people of other parts still regard it. A Chinese once conversing on the subject with the author, could only imagine of the barometer, that it was a gift of miraculous nature, which the God of Christians gave them in pity, to direct them in the long and perilous voyages which they undertook to unknown seas.

A barometer is of great use to persons employed about those mines in which hydrogen gas, or fire-damp, is generated and exists in the crevices. When the atmosphere becomes unusually light, the hydrogen being relieved from a part of the pressure which ordinarily confines it to its holes and lurking places, expands or issues forth to where it may meet the lamp of the miner, and explode to his destruction. In heavy states of the atmosphere, on the contrary, it is pressed back to its hiding places, and the miner advances with safety.

We see from this that any reservoir or vessel containing air would itself answer as a barometer if the only opening to it were through a long tubular neck, containing a close sliding plug, for then according to the weight and pressure of the external air the density of that in the cavity would vary, and all changes would be marked by the position of the moveable plug. A beautiful barometer has really been made on this principle by using a vessel of glass, with a long slender neck, in which a globule of mercury is the moveable plug.

The state of the atmosphere, as to weight, differs so much at different times in the same situation, as to produce a range of about three inches in the height of the mercurial barometer, that is to say, from

twenty-eight to thirty-one inches. On the occasion of the great Lisbon earthquake, however, the mercury fell so far in the barometers, even in Britain, as to disappear from that portion at the top usually left uncovered for observation. The uncovered part of a barometer is commonly of five or six inches in length, with a divided scale attached to it, on which the figures 28, 29, &c., indicate the number of inches from the surface of the mercury at the bottom to the respective divisions :-on the lower part of the scale the words wind and rain are generally written, meaning that when the mercury sinks to them, wind and rain are to be expected; and on the upper part, dry and fine appear, for a corresponding reason ; but we have to recollect, that it is not the absolute height of the mercury which indicates the existing or coming weather, but the recent change in its height :-a falling barometer usually telling of wind and rain ; a rising one of serene and dry weather.

The barometer answers another important purpose, besides that of a weather-glass-in enabling us to ascertain readily the height of mountains, or of any situation to which it can be carried.

As the mercurial column in the barometer is always an exact indication of the tension or pressure produced in the air around it by the weight of air above its level, being indeed, as explained in the foregoing paragraphs, of the same weight as a column of the air of equal base with itself, and reaching from it to the top of the atmospherethe mercury must fall when the instrument is carried from any lower to any higher situation, and the degree of falling must always tell exactly how much air has been left below. For instance, if thirty inches barometrical height mark the whole atmospheric pressure at the surface of the ocean, and if the instrument be found, when carried to some other situation, to stand at only twenty inches, it proves that one-third of the atmosphere exists below the level of the new situation. If our atmospheric ocean were of as uniform density all the way up as our watery oceans, a certain weight of air thus left behind in ascending would mark everywhere a change of level nearly equal, and the ascertaining any height by the barometer would become one of the most simple of calculations :—the air at the surface of the earth being about twelve thousand times lighter than its bulk of mercury, an inch rise or fall of the barometer would mark everywhere a rise or fall in the atmosphere of twelve thousand inches or one thousand feet. But

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