bubting that the magnetic virtue is referable to elec- | electric currents are constantly circulating, and these tricity. When two magnets are brought near together, the north pole of the one repels the north pole of the ather, and the same with the two south poles; but poles si apposite names attract each other. These phenomena are evidently analogous to the demonstrations of positive and negative electricity. R Artificial but permanent magnets e usually made in nearly the form d a horse-shoe, by which the two ples are brought near each other, and are connected at the extremities by a small piece of soft iron called keeper, which serves to increase the strength of the magnet when not A use. A magnet of this form is represented in fig. 8. M is the magnet, ished with a ring R at the top, by sch it may be suspended. K is the weeper, into which a hook is fixed supparting weights beneath, so as to exit the strength of the magnetic in- M K Fig. 8. In 1819, Professor Oersted of Copenhagen established most interesting relation between magnetism and volue electricity, thus laying the foundation of electromagnetism. He discovered that when a wire conductag electricity is placed parallel to a magnetic needle properly suspended, the needle will deviate from its oripal or natural direction. This deviation follows a regalar law. L. If the needle is above the conducting wire, and the puotive electricity goes from right to left, the north end of the needle will be moved from the observer. 2. If the needle is below the wire, and the positive electriety passes as before, the north end of the needle will be ved towards the observer. 3. If the needle is in the me horizontal plane with the wire, and is between We observer and the wire, the north end of it will be vated. 4. If the needle is similarly placed on the pposite side, the north end of it will be depressed. In these two experiments the needle must be very near vire. From these simple facts, Mr Oersted contales that the magnetical action of the electrical curat has a circular motion round the wire which condiets it. currents attract all other electric currents flowing in the same direction, and repel all others which are moving in an opposite direction. The electric currents flow round every magnet in the same direction in reference to its poles. For instance, if we place a magnet with its north pole pointing to the north, in the usual position of the magnetic needle, the current of electricity flows round it from west to east (that is, the direction in which the earth and other planets revolve round the sun), or, on the eastern side of the magnet, it is moving downwards, on the western side upwards, on the upper side from west to east, and on the lower side from east to west. This is found to be a uniform law. To complete the view of this doctrine, it remains only to explain the influence of the earth on the magnet, by which the needle is kept always in one position, nearly coinciding with the meridian. It is conjectured that currents of electricity, analogous to those which circulate round every magnet, are constantly flowing round the globe, as the current of electricity in a galvanic apparatus moves in an unbroken circuit from the negative to the positive pole, and from it, by the connecting wire, round again to the negative pole. The direction of these currents is inferred to be the same as has been stated with regard to artificial magnets; and it is simply by the attractions and repulsions of these terrestrial currents, bringing the currents round the needle to coincide with them, that the latter always points to the north. By means of a galvanic battery, iron may be temporarily magnetised, that is, endued with an attractive power, so long as the iron is in connexion with the seat of power in the trough. When a metallic wire of great length is coiled round the iron, forming what is called a helix, the magnetic power of the magnet is cooz 1ingly increased. The wire requires to be previously coated with silk varnished, to preven. the electric current passing from surface to surface of the metal. A magnet of this kind is usually formed in the shape of a horse-shoe, as in the case of the permanent magnet already mentioned. When susper ded so as to present the extremities downward, and the galvanic communication is established, the magnetic power is at once exerted, and a piece of iron held to the extremities will instantly be attracted and adhere. On breaking the communication with the trough or battery, the magnetic virtue is destroyed, and the piece of iron which was attracted drops. Electro-magnets have thus been made of great power; one having been formed which would sustain a weight of 2063 pounds, or nearly a ton. There is, however, no assignable limits to the power of the apparatus. The metallic wire to be made use of in this experit, should be two or three feet in length, to allow of is being bent in various directions. It is called the junctive wire. Ampère and Davy discovered two very important facts soon after Oersted had made his experiments public—namely, that the conjunctive wire elf becomes a magnet, and that magnetic properties might be communicated to a steel needle, not previously Electro-magnets, like those formed permanently, possessing them, by placing it in the electric current; sess opposite poles, one attracting and another repeland the degree of magnetic power thus communicated, ling. From this property the attempt has been made Dary showed was always proportional to the quantity to give rotation to a wheel, consisting of cross bars of electricity transmitted through it. When the con- iron; one pole attracting a bar and another repelling tive wires of two distinct galvanic batteries are it, and thus, by a rapid alternation of poles, obliging made to approach each other, they exhibit magnetic the wheel to revolve. Another plan has consisted in actions and repulsions. Two wires of copper, silver, reiterated attractions, with intervals during which the any other metal, connecting the extremities of two attraction is destroyed; in other words, the attraction alanie troughs, being placed parallel to each other, being destroyed, as relates to one bar of the wheel, that and suspended so as to move freely, immediately attract bar is liberated and allowed to go on, while the attracand repel each other, according as the directions of the tion is being exerted on the next bar. currents of electricity flowing through them are the me or different. Upon this experiment is founded the most plausible theory of magnetism, namely, that arises from the attractions and repulsions of currents of electricity, constantly circulating round every get. This is conceived to explain the reason why the magnetic needle places itself at right angles to a wire conducting electricity, namely, that the current ping along the wire may coincide with that circulating round the magnet. From these and other experiments, it seems clearly proved that electricity and magnetism are identical. A permanent magnet is supposed to be thus constituted: It is a mass of iron or steel, round the axis of which Electro-Magnetic Machines.-The possibility of moving small pieces of mechanism by electro-magnetic action, to which allusion has just been made, has been known for a number of years; but as far as we have heard, the principle was not applied practically till 1837, when it was adapted by Professor Jacobi of St Petersburg to the propulsion of a small vessel on the Neva. Under the auspices of an imperial commission, the first attempt was made on the 25th of September 1838. A galley or boat, 28 feet in length and 7 in breadth, was provided with paddles similar to those of a steam-vessel, and the apparatus was put on board. The action was produced from 320 pairs of plates, arranged along the sides of the boat, and room was left letter, although the idea had to travel through four | It is the intention of the Great Western Railway Commiles of wire. In the transmission of the electric influence through the wires of this or any other apparatus, distance is of no consequence as respects time, for electricity is supposed, with some degree of probability, to travel with the velocity of light, or 192,000 miles in the space of a second. In point of fact, therefore, no longer time would be occupied in transmitting intelligence to the uttermost ends of the earth, than would be required for sending it across a room or a table. Distance is a matter for consideration only as regards expenditure of galvanic force. The electric agency has a tendency to weaken in its progress, according to circumstances, and this must necessarily be provided for by increasing the number of batteries to the desired amount and power. It has been supposed that the difficulty of perfectly isolating and preserving the wires from injury in their course, would be an insuperable bar to their establishment on an effective footing; but fears need be no longer entertained on this score. Each of the four wires in the above apparatus, is wrapped round with a well-rosined thread, and the whole are then tied together with a cord possessing a similar coating, so as to present the appearance of a tightly-bound rope. This it is proposed to place in a small iron tube, like that used for bringing gas into houses, and the tubes, united to any length, are laid below the ground, or in a wooden case on the surface, to preserve them from injury. Yet another difficulty here presents itself. What if the rope, or any particular wire, should be fractured somewhere in its course? How would the precise point of injury be discoverable? This the professor has likewise provided for, as far as it possibly He proposes that there shall be a signal-case val of every few miles along the whole line, and therefore should any injury be sustained by the wires, it will be speedily discovered in what portion it has taken place, and a new and complete section of rope inserted in connexion with the other pieces. To avoid a very remote chance of delay in the transmission of intelligence from this cause, it would be easy to lay two sets of wires, one of which could be employed while the other was in course of being repaired. can he pany to carry the tube along the line as fast as the completion of the rails takes place, and ultimately throughout the whole distance to Bristol. The machinery, and the mode of working it, are so exceedingly simple, that a child who could read would, after an hour or two's instruction, be enabled efficiently to transmit and receive information.' It being thus ascertained, by practical working, that the electric telegraph can perform all that its designers have proposed, it only remains that it should be spread in different directions over the country, or at the least laid in communication from London along the great lines of thoroughfare. The method of working the apparatus will be readily understood. At each extremity of the line of ropefor it would work both ways-there would be an office for receiving and communicating intelligence, at a price conformable to the extent of the message. Being dispatched from one end, the communication would be instantaneously received at the other by an officiating clerk, and forthwith made known by a note to the party concerned. Thus, intelligence of the rise and fall of stocks, foreign news, orders for goods, or any other species of communication of an urgent nature, might, with the utmost facility, and at a trifling cost, be transmitted to any imaginable distance.” Still more lately the galvanic principle has been ap plied to the regulation of clocks, with a view to preserve a uniformity of motion in all the clocks in a town, or in a public office, to which wires may be led. The manner in which this is accomplished is now (or was lately) exhibited at the Polytechnic Institution in London. ELECTRO-MAGNETISM. BEFORE noticing this electric quality, it seems necessary to give a short explanation of Magnetism, or the properties of the Magnet. Anciently, there was found in Magnesia, in Asia, a certain kind of iron ore, in which the remarkable property was discovered of attracting other kinds of iron or steel; this ore afterwards reThe capabilities of the principle have been fully ceived the name of loadstone, but from Magnesia, the tested in a practical manner on the line of the Great place in which it was originally found, we derive the Western Railway. In September 1839, when the wires terms magnet and magnetism. Latterly, loadstone ore of the electric telegraph were carried as far as West has been discovered in Siberia, Sweden, Piedmont, the Drayton, a distance of fifteen miles, the following ac- kingdom of Naples, and various places in North Ame count was given of it in one of the London papers:- rica. This magnetic iron ore, which is of an excellent The space occupied by the case containing the ma-quality for making steel, is of a dark colour, and genechinery (which simply stands upon a table, and can be rally occurs crystallised in the form of regular octaremoved at pleasure to any part of the room) is little hedrons; its attractive quality is strengthened by exmore than that required for a gentleman's hat-box. The posure to the air. It has likewise been found that telegraph is worked by merely pressing small brass meteoric stones, which are composed of iron and nickel, keys (similar to those of a keyed bugle), which, acting possess a strong magnetic virtue resembling the loadby means of galvanic power upon various hands placed stone of the earth. upon a dial plate at the other end of the telegraphic line, as far as now opened, point not only to each letter of the alphabet (as each key may be struck or pressed), but the numerals are indicated by the same means, as well as the various points, from a comma to a colon, with notes of interrogation and interjection. There is likewise a cross (+) upon the dial, which indicates that where this key is struck, a mistake has been committed in some part of the sentence telegraphed, and that an erasure is intended. To a question-such, for instance, as the following: "How many passengers started from Drayton by the ten o'clock train?"-the answer could Independently of attracting iron, magnets possess the be transmitted from the terminus to Drayton and back extraordinary property of polarity. When a small magin less than two minutes. This was proved on Satur-netic bar is poised at the centre, so as to be free to move day. This mode of communication is only completed as far as West Drayton station, which is about 13 miles from Paddington. There are wires (as may be imagined) communicating with each end, thus far completed, passing through a hollow iron tube, not more than an inch and a half in diameter, which is fixed about six inches above the ground parallel with the ilway, and about two or three feet distant from it. Although the ancients were acquainted with the attractive property of the loadstone, it was left for the moderns to discover that the property could be communicated to the iron which the magnetic stone attracted. Since this important discovery was made, artificial magnets, composed of bars or slips of iron, have been easily constructed; and these possess all the attractive virtues of the loadstone itself. Besides iron, a few other metals are susceptible of being attracted, such as pure cobalt and nickel; but the power is weak and of no avail in the arts. in any direction, one end points towards the north pole of the earth and another towards the south. It is true, there are variations in the direction at different parts of the globe, but with these slight exceptions, the magnetic needle, as it is called, offers one point to the north and another to the south. Hence the application of the compass to navigation. (See the Article OCEAN.) From what will be immediately mentioned, no room is left for doubting that the magnetic virtue is referable to elec- | electric currents are constantly circulating, and these tricity. When two magnets are brought near together, the north pole of the one repels the north pole of the other, and the same with the two south poles; but poles of opposite names attract each other. These phenomena are evidently analogous to the demonstrations of positive and negative electricity. Artificial but permanent magnets are usually made in nearly the form of a horse-shoe, by which the two poles are brought near each other, and are connected at the extremities by a small piece of soft iron called the keeper, which serves to increase the strength of the magnet when not in use. A magnet of this form is represented in fig. 8. M is the magnet, furnished with a ring R at the top, by which it may be suspended. K is the keeper, into which a hook is fixed supporting weights beneath, so as to exhibit the strength of the magnetic in-fluence. M R K currents attract all other electric currents flowing in the same direction, and repel all others which are moving in an opposite direction. The electric currents flow round every magnet in the same direction in reference to its poles. For instance, if we place a magnet with its north pole pointing to the north, in the usual position of the magnetic needle, the current of electricity flows round it from west to east (that is, the direction in which the earth and other planets revolve round the sun), or, on the eastern side of the magnet, it is moving downwards, on the western side upwards, on the upper side from west to east, and on the lower side from east to west. This is found to be a uniform law. To complete the view of this doctrine, it remains only to explain the influence of the earth on the magnet, by which the needle is kept always in one position, nearly coinciding with the meridian. It is conjectured that currents of electricity, analogous to those which circulate round every magnet, are constantly flowing round the globe, as the current of electricity in a galvanic apparatus moves in an unbroken circuit from the negaFig. 8. tive to the positive pole, and from it, by the connecting In 1819, Professor Oersted of Copenhagen established wire, round again to the negative pole. The direction a most interesting relation between magnetism and vol- of these currents is inferred to be the same as has been taic electricity, thus laying the foundation of electro-stated with regard to artificial magnets; and it is magnetism. He discovered that when a wire conducting electricity is placed parallel to a magnetic needle properly suspended, the needle will deviate from its original or natural direction. This deviation follows a regular law. ΔΙΑ 1. If the needle is above the conducting wire, and the positive electricity goes from right to left, the north end of the needle will be moved from the observer. 2. If the needle is below the wire, and the positive electricity passes as before, the north end of the needle will be moved towards the observer. 3. If the needle is in the same horizontal plane with the wire, and is between the observer and the wire, the north end of it will be elevated. 4. If the needle is similarly placed on the opposite side, the north end of it will be depressed. In these two experiments the needle must be very near the wire. From these simple facts, Mr Oersted concludes that the magnetical action of the electrical current has a circular motion round the wire which conducts it. simply by the attractions and repulsions of these terrestrial currents, bringing the currents round the needle to coincide with them, that the latter always points to the north. By means of a galvanic battery, iron may be temporarily magnetised, that is, endued with an attractive power, so long as the iron is in connexion with the seat of power in the trough. When a metallic wire of great length is coiled round the iron, forming what is called a helix, the magnetic power of the magnet is coodingly increased. The wire requires to be previously coated with silk varnished, to prevent the electric current passing from surface to surface of the metal. A magnet of this kind is usually formed in the shape of a horse-shoe, as in the case of the permanent magnet already mentioned. When susper ded so as to present the extremities downward, and the galvanic communication is established, the magnetic power is at once exerted, and a piece of iron held to the extremities will instantly be attracted and adhere. On breaking the communication with the trough or battery, the magnetic virtue is destroyed, and the piece of iron which was attracted drops. Electro-magnets have thus been made of great power; one having been formed which would sustain a weight of 2063 pounds, or nearly a ton. There is, however, no assignable limits to the power of the apparatus. The metallic wire to be made use of in this experiment, should be two or three feet in length, to allow of its being bent in various directions. It is called the conjunctive wire. Ampère and Davy discovered two very important facts soon after Oersted had made his experiments public-namely, that the conjunctive wire itself becomes a magnet, and that magnetic properties might be communicated to a steel needle, not previously possessing them, by placing it in the electric current; and the degree of magnetic power thus communicated, Davy showed was always proportional to the quantity of electricity transmitted through it. When the conjanetive wires of two distinct galvanic batteries are made to approach each other, they exhibit magnetic attractions and repulsions. Two wires of copper, silver, or any other metal, connecting the extremities of two galvanic troughs, being placed parallel to each other, and suspended so as to move freely, immediately attract and repel each other, according as the directions of the currents of electricity flowing through them are the Electro-Magnetic Machines.—The possibility of movsame or different. Upon this experiment is founded ing small pieces of mechanism by electro-magnetic the most plausible theory of magnetism, namely, that action, to which allusion has just been made, has been it arises from the attractions and repulsions of cur-known for a number of years; but as far as we have rents of electricity, constantly circulating round every fanguet. This is conceived to explain the reason why the magnetic needle places itself at right angles to a wire conducting electricity, namely, that the current passing along the wire may coincide with that circulating round the magnet. Electro-magnets, like those formed permanently, possess opposite poles, one attracting and another repelling. From this property the attempt has been made to give rotation to a wheel, consisting of cross bars of iron; one pole attracting a bar and another repelling it, and thus, by a rapid alternation of poles, obliging the wheel to revolve. Another plan has consisted in reiterated attractions, with intervals during which the attraction is destroyed; in other words, the attraction being destroyed, as relates to one bar of the wheel, that bar is liberated and allowed to go on, while the attraction is being exerted on the next bar. From these and other experiments, it seems clearly proved that electricity and magnetism are identical. A permanent magnet is supposed to be thus constituted: -It is a mass of iron or steel, round the axis of which heard, the principle was not applied practically till 1837, when it was adapted by Professor Jacobi of St Petersburg to the propulsion of a small vessel on the Neva. Under the auspices of an imperial commission, the first attempt was made on the 25th of September 1838. A galley or boat, 28 feet in length and 7 in breadth, was provided with paddles similar to those of a steam-vessel, and the apparatus was put on board. The action was produced from 320 pairs of plates, arranged along the sides of the boat, and room was left for twelve persons. Owing to imperfections in the arrangements, the attempt to propel the vessel with its burden of apparatus and passengers was less successful than was expected; nevertheless, the professor succeeded in making the boat proceed against the stream, and the speed attained in still water was three English miles per hour. In this and other efforts of Professor Jacobi, his plan consisted in rapidly reversing the poles during the action. While Jacobi was busy with his experiments in Russia, mechanicians in England and America were pursuing a similar course of investigation. In March 1837, Mr Davenport exhibited at New York an electro-magnetic machine of considerable power; and more lately great advances in the art of electro-magnetic motion were made in Germany by Stoehrer of Leipsic and others, on the principles of Jacobi ; and in some instances with remarkable success. According to the accounts given of their experiments, it appears that the power of the electro-magnetic machines is increased in proportion to the squares of the number of the elements of the batteries. We regret to say that this, like all other enunciations on the subject, wants the confirmation of practical men; and up till the period we now write (December 1841) no trustworthy account has reached us, as respects either the ratio of the magnetic power or the expense at which it can be obtained. Machines constructed on the principle of alternate attraction and repulsion being liable to several objections, those in which a reiterated series of attractions are employed are more likely to answer the end of practical working. A machine of this kind was contrived by Mr R. Davidson, of Aberdeen, in 1839, and brought into notice by the following letter from Professor Forbes of King's College, in that city, to Professor Faraday (Oct. 7, 1839), which was published in the London and Edinburgh Philosophical Journal :— "Having seen a notice from Mr Jacobi sent by you to the London and Edinburgh Philosophical Magazine, regarding the success of his experiments on the production of a moving power by electro-magnetism, I am sure it will give you pleasure to know that a countryman of our own, Mr Robert Davidson, of this place, has been eminently successful in his labours in the same field of discovery. For, in the first place, he has an arrangement by which, with only two electromagnets, and less than one square foot of zinc surface (the negative metal being copper), a lathe is driven with such velocity as to be capable of turning small articles. Secondly, he has another arrangement by which, with the same small extent of galvanic power, a small carriage is driven, on which two persons were carried along a very coarse wooden floor of a room. And he has a third arrangement, not yet completed, by which, from the imperfect experiments he has made, he expects to gain very considerably more force from the same extent of galvanic power than from either of the other two. The first two of these arrangements were seen in operation by Dr Fleming, Professor of Natural Philosophy in this University, and myself, some days ago; and there remains no doubt on our minds, that Mr Davidson's arrangements will, when finished, be found available as a highly useful, efficient, and exceedingly simple moving power. He has been busily employed for the last two years in his attempts to perfect his machines, during all which time I have been acquainted with his progress, and can bear testimony to the great ingenuity he has shown in overcoming the numberless difficulties he has had to encounter. So far as I know, he was the first who employed the electro-magnetic power in producing motion by simply suspending the magnetism without a change of the poles. This he accomplished about two years ago. About the same time, he also constructed galvanic batteries on Professor Daniell's plan, by substituting a particular sort of canvass instead of gut, which substitution answers perfectly, is very durable, and can be made of any form or size. And, lastly, he has ascertained the kind of iron, and the mode of working it into the best state for producing the strongest magnets with certainty. The first two machines, seen in operation by Dr Fleming and myself, are exceedingly simple, without, indeed, the least complexity, and therefore easily manageable, and not liable to derangement. They also take up very little room. As yet, the extent of power of which they are capable has not been at all ascertained, as the size of battery employed is so trifling and the magnets so few; but from what can be judged by what is already done, it seems to be probable that a very great power, in no degree even inferior to that of steam, but much more manageable, much less expensive, and occupying greatly less space, if the coals be taken into account, may be obtained. In short, the inventions of Mr Davidson seem to be so interesting to railroad proprietors in particular, that it would be much for their interest to take up the subject, and be at the expense of making the experiments necessary to bring this power into operation on the great scale, which, indeed, would be very trifling to a company, while it is very serious for an individual by no means rich, and who has already expended so much of his time and money for the mere desire of perfecting machines which he expected would be so beneficial to his country and to mankind. For it deserves to be mentioned, that he has made no secret of his operations, but has shown and explained all that he has done to every one who wished it. Ilis motives have been quite disinterested, and I shall deem it a reproach to our country and countrymen if he be allowed to languish in obscurity, and not have an opportunity afforded him of perfecting his inventions and bringing them into operation, when they promise to be productive of such incalculable advantages." The following notice of Mr Davidson's operations afterwards appeared in the Aberdeen Constitutional newspaper, November 6, 1840: "Mr Davidson's invaluable invention is now set down, by common consent, as the desideratum that has been wanting to perfect the power of locomotive agency. Several thousands have visited the exhibition, among whom was Professor Hamel, whose opinion was decidedly favourable to the principle. At the meeting of the British Association of Science, held at Glasgow the other week, Professor Jacobi read a paper on the power of electro-magnetism, which seemed to point to some great improvement in the science; but the principle on which Mr Davidson's machine proceeds is altogether different from that on which Jacobi's experiments were made. Professor Jacobi produces motion by changing the poles of the magnets, Mr Davidson by cutting off the galvanic current at given points the power alternating, as the rotation proceeds, from a neutralised magnet to a newly charged one. In both experiments, it has been clearly demonstrated that the power of the magnet is increased by increasing the diameter, and adding to the length of the helix. The power may be also increased by increasing the sizes of the bars. On the principle of changing the poles of the magnet, the advantages of this increase of power could not be made so fully available as on the principle of neutralising the magnets-there being in the one case a back action, which retards the momentum power, while in the other the rotation is constant, which tends to increase that power. According to the proportions assigned by Jacobi, an almost indefinite amount of power may be obtained by increasing the diameter of the rods, and the quantity of wire or helix; this, too, constitutes an index of that power so simple and practicable, that it may be regulated to a fraction." In concluding the subject of Electricity, in the various methods in which it is artificially demonstrated, it is necessary to remind the reader that the science, in all its departments, is still in its infancy, and offers great scope for the enterprise of ingenious students. Printed and published by W. and R. CHAMBERs, Edinburgh. Sold also by W. 8. Orr and Co., London. CHAMBERS'S INFORMATION FOR THE PEOPLE. CONDUCTED BY WILLIAM AND ROBERT CHAMBERS, EDITORS OF CHAMBERS'S NEW AND IMPROVED SERIES. METEOROLOGY-THE WEATHER. PRICE ląd. METEOROLOGY is the great ocean of air, as we may call it, is far from being science of the Wen- of a uniform density throughout its mass. At and near ther, and treats of the the level of the sea it is most dense, in consequence phenomena which oc- of the pressure above. As we ascend mountains, or cur in the atmosphere, in any other way penetrate upwards, the air becomes their causes and ef- gradually less dense; and so thin is it at the height of fects. Men in all ages three miles, as, for instance, on the summit of Montof society have been Blanc, one of the Alps, that breathing is there perled, by motives of ne- formed with some difficulty. Beyond this limited cessity or comfort, to height, the density of the air continues to diminish; study the indications of and at the elevation of about forty-five miles it is bethe weather in the dif-lieved to terminate. So dense are the lower in proporferent appearances of tion to the higher regions, that one-half of the entire the skies. The mariner, body of air is below a height of three miles, the other the shepherd, the hus- half being expanded into a volume of upwards of forty bandman, and the hunter, have the strongest motives to miles. examine closely every varying appearance which may precede more important changes. The result of these observations forms a body of maxims, in which facts are often stated correctly, but mixed with erroneous deductions and superstitious notions, such as the credality of ignorant people always renders them ready to adopt. Hence the disposition to refer the ordinary changes of the weather to the influence of the moon, and even the stars; and to look for signs of approaching convulsions, even in the moral world, in horrid comets and strange meteors. The progress of science, which tends to separate the casual precursors from the real causes of phenomena, refutes these false reasonings, dissipates the empty terrors to which they give rise, and aims, by more patient, long-continued, and wideextended observations, to deduce the general rules by which the phenomena of the atmosphere appear to be regulated. The atmosphere is an invisible aëriform fluid, which wraps the whole earth round to an elevation of about forty-five miles above the highest mountains. This * Conversations Lexicon. The extreme height of the atmosphere is not observable from the situation in which we are placed on the earth. Our eye, on being cast upwards, perceives only a vast expanded vault, tinted with a deep but delicate blue colour; and this in common language is called the heavens or the sky. The blueness so apparent to our sense of sight is the action of the rays of light upon the thin fluid of the upper atmosphere, and the brightness is in proportion to the absence of clouds and other watery vapours. In proportion as the spectator rises from the surface of the earth, and has less air above him, and that very rare, the blue tint gradually disappears; and if he could attain a height at which there is no air, say fifty miles above the level of the sea, the sky would appear dark or black. Travellers who have ascended to great heights on lofty mountains, describe the appearance of the sky from these elevated stations as dark or of a blackish hue. The atmosphere exerts a certain pressure on all objects, the degree of pressure being of course in proportion to the height of the atmosphere at the spot. The part at which the pressure is greatest is at the level of the sea, for there the atmosphere is highest. The pressure at that level is ordinarily computed to be about fifteen pounds on every square inch. At every step upward from the level of the ocean, the burden of the superincumbent mass lightens, and at the height of three miles, one-half of the weight is lost. The Barometer. The pressure of fifteen pounds to the square inch at the level of the sea, is found by experiment to be equal to the weight of a column of mercury of thirty inches in height; and the fact of such being the case has suggested the construction of an instrument to measure the atmospheric pressure at different points and in various circumstances. This instrument is called the barometer, a compound from two Greek words, signifying weight and measure. The barometer in common use consists of a narrow glass tube upwards of thirty inches in length, and bent upwards at its lower extremity, as represented in fig. 1. The mercury is introduced into the |