great, any more than a wet sponge would be deranged by pressing it any depth in water." (Mechanical Philosophy,' vol. iii. p. 541.)

The temperature of air, as already noticed, influences its elastic force. We have every reason to conclude, that the principal properties of this and all other gases are a consequence of the presence of heat, though we do not know what the latter is. It is probable that air would become first liquid, and then solid, if it could be made sufficiently cold. Like all other substances, air gives out heat when it is compressed, that is, raises the temperature of surrounding bodies, and vice versa. This is strikingly illustrated by the fact that tinder can be set on fire when the air contained in a brass cylinder is suddenly and violently compressed by a piston.

From careful experiments it appears, that air and all other gases, as well as vapours, and also all mixtures of gases and vapours, obtain an increase of elastic force for every increase of temperature, and expand therefore, if expansion be possible, in the vessel which contains them. The quantity of this expansion, when the temperature passes from the freezing to the boiling point of water (that is, from 32° to 212° of Fahrenheit's, from 0° to 80° of Réaumur's, and from 0° to 100° of the Centigrade, thermometers), is 366 parts out of a thousand of the bulk which it had at the freezing point. That is, in the apparatus indicated in the preceding part of this article, form a graduated scale along B A, and suppose that B E contains a thousand parts, and that, the upper air being removed, as much mercury is poured in above E F as will cause the membrane EF to stand at E, when the temperature of the air is at the freezing point of water. Then, if the air be gradually heated from the freezing to the boiling point of water, either more mercury must be poured in, or the membrane with the superincumbent mercury will rise through 366 divisions more of the scale, and E will stand at 1366. And this dilatation is uniform: that is, whatever expansion arises from an increase of 12° of temperature, half as much arises from an increase of 6°, twice as much from one of 24°, and so on. This remarkable law, which holds, with perhaps a slight variation, at very high and very low temperatures, was discovered nearly at the same time by Dalton in England and Gay-Lussac in France. Now, in Fahrenheit's thermometer there are 212°-32°, or 180° between the boiling and freezing points of water; 80° in Réaumur's; and 100° in the Centigrade. Consequently, the whole increase of bulk, or, will give 306 and, for the variations of bulk corresponding to a rise of one degree of temperature on each of the three thermometers; that is, about, and, respectively. But in applying these rules, it must be recollected that, in taking Fahrenheit's thermometer, for example, the expansion is of the bulk which it had at the freezing point. Suppose, for instance, we have a bulk of air which occupies 1000 cubic inches at the temperature of 62° Fahrenheit, and we wish to know how much it would occupy under the same pressure at 82° of the same. The first temperature is 30° above the freezing point. Now, suppose a certain volume of air to consist of 491 parts at the freezing point; then it is clear that, as air expands of its bulk for each degree, this volume at 62° will have become 491+30 521 parts. Similarly, at 82° it will be 491 + 50541 parts. Hence, as 521 (the volume at 62°): 1000 (also at 62°), so is 541 (the volume at 82°): 10383, which is the bulk required at 82°.

On the properties of air with regard to other bodies, we may notice that probably there is a slight adhesion of air to many, if not to all, surfaces. A small needle may be made to swim on water, and in this state the water evidently retires from around it, leaving it, as it were, suspended over a hollow in the fluid. This is attributed to the adhesion of a coat of air, which, with the iron, makes the whole specifically lighter than the water. Recent experiments on the pendulum, the most delicate of all philosophical instruments, have led some to suspect, that in addition to the resistance of the air, a slight coating of this substance travels with the pendulum, and thereby causes an irregular addition to its weight. [PENDULUM.]

The air is a permanent gas, incapable of being reduced to the liquid state by cold or pressure. It is also, like most gases, perfectly colourless, especially when we look through small quantities of it; although, if we notice the effect produced by large masses of it, we may consider it to be a coloured gas. Thus, the blue colour of the sky is probably merely the colour of the air seen through a length of about 45 miles. Hence, it has been observed by those who have ascended about 5 miles from the earth's surface, when they have left much more than half the atmosphere behind them, that the sky appears of a dark inky hue, owing to the very small reflection and dispersion of the light, while the blue colour no longer appears above, but below them. Similarly, the blue colour of distant hills is owing to the same cause.

In this article we have considered only the chemical and mechanical properties of air. The constitution of the whole mass will come under the article ATMOSPHERE. To complete the subject, refer to OXYGEN, RESPIRATION, COMBUSTION, VENTILATION, ACOUSTICS, AERODYNAMICS, and also to the Elements of Chemistry,' by Professor Miller, of King's College, London, and to the Cours de Chimie,' by Regnault. AIR, in music, signifies 'Melody;' the terms are synonymous, it being understood that by both words is meant a succession of single sounds in measured time. The word Air was used in this sense nearly three centuries ago; but it is not now known why such an application of a familiar word was first adopted.

Rousseau says that the name of air is given to all melodies, to dis

tinguish them from recitative. M. Suard, in the 'Encyclopédie Méthodique,' offers the following definition :--a piece of music, composed of a certain number of melodious phrases, united in a regular symmetrical form, and terminating in the key in which it began. Sulzer has followed M. Suard; so has Pietro Lichtenthal; but, without objecting to his definition, we consider the common and simple one the best,-namely, that succession of single sounds, regulated by the laws of musical rhythm, which constitutes what, in homely language, is called a tune. [RHYTHM.]

Air, or melody, is, allowedly, the most important of the constituents of music. A composition may be replete with learned and ingenious harmony, may abound in fugue, in imitation, and all the contrivances of science, but without good melody will never appeal to the heart, and seldom afford any gratification to the ear. Haydn carried this opinion so far as to say, "Let your air be good, and your composition, whatever it may be, will possess beauty, and must certainly please." Air is in music what design and outline are in the sister art of painting: harmony is the filling up, and the colouring.

The Greeks had many kinds of airs, which they called nomes, or songs; and we learn from the work of Philodemus on Music, recovered from the ruins of Herculaneum, that every trade and occupation had its Nouol, or appropriate airs, which were played or sung to the workmen while they laboured.

The various kinds of airs, instrumental as well as vocal, will be found under their different heads. [ALLEMANDE; BARCAROLLE; &c.] In music composed for the theatre, and which is constantly introduced into the concert-room, are the following varieties of air, designated by Italian denominations, viz.—

The Aria di Carattere (characteristic air'), which is distinguished by force and energy of expression, and by dramatic effect. The Aria Parlante (' speaking air'), which is rather declaimed than sung, and is best suited to the buffo or comic performer. The Aria di Cantabile (singing air'), a tender, pathetic air, calling forth the expression and taste of the singer. The Aria di Bravura (dashing air'), an air in which the performer displays his powers of execution, and seeks rather to astonish than please.

AIR-CUSHIONS. The mechanical application of common air, in respect of its pressure or elasticity, has been greatly extended within the last few years. Provided a mass of air can be confined within a given receptacle, and that receptacle be of an elastic or yielding character, the air assumes many of the qualities of a soft stuffing or padding, when its quantity is small compared with the size of its envelope; but when the quantity is as great as can be introduced without bursting the envelope, the air becomes nearly equivalent to a solid body. So long as the means were wanting for conveniently making air-tight cloth vessels, this principle was slenderly applied; but the use of caoutchouc or India-rubber, as a glutinous varnish, has developed many ingenious contrivances for this object. When a bag has been made of such material, rendered also air-tight by somewhat similar means at the seams, air may be passed into it as a substitute for more solid materials. In practice there are some very neat arrangements adopted in effecting this. Temporary air-seats or cushions are made by forming a bag of air-tight cloth, perfectly enclosed at every part except one corner, where is inserted a small tube and stop-cock, capable of effecting or preventing communication from the interior to the exterior. The cock being opened, and the tube applied to the mouth, air is blown into the cushion, until it expands to the desired degree of fulness; the cock is then closed, and the air remains imprisoned. When not in use, such a cushion can have the air expressed from it, and may then be folded up into a small space.

It is obvious that seats, cushions, pillows, and beds of various kinds, having a similar object in view in relation to softness, fulness, and elasticity, may be made by similar means. When the quantity of air included in an envelope is greatly increased, it may be made the means of producing actual pressure in a more equable way than by any solid bodies. Thus, an air-tight bandage was invented a few years ago, the object of which was to form a wrapper to a human limb under surgical treatment. There is a bandage with straps fitted for being tied round the limb; and when so tied, air is breathed into an air-tight envelope to the bandage, through a small tube provided with a pipe. By increasing or lessening the quantity of air impelled, the pressure of the bandage on the limb may be made so small as to be scarcely perceptible, or may on the other hand be made even painfully close; but in either case it will be equable in every part.

A patent has been obtained, by Mr. Walton, for certain modifications in the mode of preparing air-bags for beds and similar purposes. Hollow balls of India-rubber, filled with air, are enclosed within external coverings so as to form a sheet of balls; and many such sheets may be piled one on another to form a bed or mattrass of any required thickness. Such an arrangement would, however, be very complicated and expensive.

Further illustrations bearing on this subject will be met with under LIFE-BOAT and WATERPROOF COMPOSITIONS. AIR ENGINES. engines which should have the power of steam-engines without the use Many attempts have been made to produce of steam. The compression or the rarefaction of air, brought about in some one of many different ways, is the agent relied upon for producing a moving force. In 1840 Mr. Stirling patented such a machine, and

and by the time the piston has descended so much farther that the additional elastic force acquired from compression will suffice to lift the valve A, the latter will open, and the air will rush out. This continues until the piston has quite returned to B. That is to say, after every stroke of the piston, the air in the vessel has only six-sevenths of the density which it had before the stroke, since the air contained in six measures is expanded into seven by the rise of the piston. Therefore, at the end of the second stroke the density is of, or 3, that is, 36 measures of common air would weigh as much as 49 of the air we have now got inside the vessel. At the end of the third stroke the density is of, or . Without going farther, suffice it to say, that at the end of the twentieth stroke, the density of the rarefied air is about; and at the end of 100 strokes, it would take about five million of measures of the rarefied air to weigh as much as one of common air. But long before this time a limit would be put to the exhaustion, in the present state of the apparatus. The air in the vessel cannot escape into the tube unless it has force sufficient to lift up the valve B; which after a certain number of strokes will not be the case, for the elastic force of the air diminishes in the same proportion as its density, being at first fifteen pounds to the square inch; so that by the time the density is reduced to, the valve, if it present a surface of one square inch, will not rise, if it be so heavy as half an ounce. Let us, then, suppose B to be fastened to the piston by a loose string, so long that it becomes tightened just before the piston reaches its greatest height. The string will then open the valve, and the rarefaction will take place as usual.

The condensing instrument will now be easily understood. Let the piston be raised, the valves will then be open; but the moment the piston begins to descend, the rush of air outwards will shut c, and the whole of the air in the tube will be forced into the vessel, which admits it, since D opens inwards. If this be done quickly, so that hardly any air escapes, seven measures of air, after the stroke, will occupy the space filled by six measures before it, so that the density of the air in the vessel will be ; or six measures of condensed air will weigh as much as seven of common air. When the piston returns, air rushes in through c, and presses the valve D, which nevertheless, unless made too heavy, does not open, because it is pressed with a greater force from within. In every succeeding stroke an additional measure of common air is added to the stock already contained in the vessel. At the end of the second stroke the density is, at the end of the third, and so on. Every succeeding stroke will be more difficult,

exhausting syringe takes place in geometrical progression. It would take 30,000,000 of strokes, all but one, to produce a condensation, the corresponding rarefaction to which is gained in a hundred. It is needless to say, that no materials that we could put together would bear such a pressure, and no force that we could exert would create it. The exhausting syringe, as above described, is, in principle, the common air-pump. We shall now proceed to describe Cuthbertson's air-pump, containing the most recent material improvements. The circular plate or table at the top is of metal or of glass ground to a perfect plane surface, on which is placed an inverted glass jar, from which the air is to be exhausted, called the receiver, the bottom of which is also carefully ground: so that if the plate be slightly smeared with grease and the receiver placed upon it, the junction of the two is air-tight. The hole in the middle of the plate is the end of a tube, which extends vertically downwards, until, curving at the bottom, it passes through the front beam below the barrels, with the interior of each of which it communicates. These barrels are exhausting syringes, the construction of which will presently be more particularly described: they are worked by rack-work, communicating with a cogwheel and handles, space for the racks to play being cut in the upper wood-work of the apparatus. On the left are the gauges for ascertaining the degree of exhaustion obtained, and at A is a place for a PEAR-GAUGE. See also SYPHON-GAUGE, as we shall here only describe the most common, the barometer-gauge. The box attached to the under beam on the left contains mercury, out of which rises a tube and a graduated scale, as in the barometer. This tube passes through the higher wood-work, and also ends in the orifice which is in the middle of the plate, so that the communication being free, the air in the receiver, and that in the tube above the mercury, are in the same

the degree of rarefaction. For example, suppose the common barometer to be 30 inches high, while the barometer-gauge of the air-pump stands at 20 inches. If the vacuum were complete, the barometergauge would be a common barometer, and would stand at 30 inches: but as it stands only at 20 inches, the pressure of the air in the receiver is equivalent to 10 inches of mercury, or one-third of that of the exterior air. Therefore the density of the air in the receiver is one-third of that of the exterior air, or two-thirds of the air have been removed.

The preceding cut shows a section of the piston rod, as well as of the barrel. The tube m comes from the receiver, and air can be admitted by it into the barrel, when the rod gg is raised. The rod gg passes into the piston-rod (which is hollow), and works stiffly in it, being however unconnected with it except by friction. This rod consists of two parts, above and below L, the latter of which is not thick enough to fill the orifice in which it plays. But when the piston descends, the conical juncture of the thicker and thinner parts is brought upon this orifice, and shuts it close. After this, and during the rest of the descent, the hollow piston-rod slides downwards upon the rod gg. As soon as the piston begins to ascend, the rod gg is raised with it, owing to the friction, so far as the nut o will let it rise, after which the piston-rod slides up gg. We have here the lower valve of the

n

exhausting syringe, shut during the descent of the piston, open during the ascent, and not opened by the force of the air from underneath, so that the functions of the string which we supposed in our first exemplification are performed. A little higher up the barrel we find the piston, as better shown in the adjoining figure. The external part is a partial piston not connected with the piston-rod, but fitting closely to the barrel. The piston-rod, when rising, fits this exactly, renders it airtight, and causes it also to rise. But when the piston-rod is descending, it will not cause the descent of the exterior, and, as we have called it, partial, piston, until the projecting shoulders aa (in the figure) come upon it; and, as these shoulders do not go all the way round, the piston in descending is not air-tight. This apparatus supplies the place of the upper valve, being air-tight in the ascent, but not so in the descent. Looking above the piston, we find that its rod works in metal shoulders, the interval between which is occupied by stiff leathers. The space above the leathers opposite to a is filled with oil, which is communicated slowly to the leathers, and also to the barrel beneath. From the latter, however, it is immediately expelled by the rise of the piston, which forces it, as well as the air in the barrel, through the channel aa. The oil and the air then force up the rod in the cavity R, which rod, working in collars, answers the purpose of a valve. The oil is there lodged, until it is collected in sufficient quantity to flow again into the reservoir at T. The air escapes into the exterior atmosphere. Having shown that we have here an under valve shut during the descent, and open during the ascent, with an upper valve open during the descent and shut during the ascent, we need not repeat the manner in which the rarefaction is produced. We have only further to notice, that a branch from the main tube which enters the receiver is carried through the under wood-work in front, and emerges at B. It is here stopped by a screw; but when the operator desires to restore the air under the receiver, he opens this screw, upon which the communication between the exterior atmosphere and the receiver is restored, and the air rushes in. In the perspective figure, a cross bar, in which the upper parts of the barrels are inclosed to strengthen them in their position, is omitted for the sake of clearness.

We give in the following figure a representation of a more portable and less expensive species of air-pump, which, after what we have said, will need no description.

The small plate behind the receiver is for another small receiver, in which a gauge is placed. This guage is nothing more than a common barometer, which falls with the diminution of pressure from the air in the receiver, in the same way as the common barometer when the pressure of the exterior air is lessened by a change of weather.

The following experiments are among the most common of those shown with the air-pump :

1. If the receiver be open at both ends, and the upper orifice be stopped by the hand,-on exhaustion the pressure of the exterior air will prevent the removal of the hand. If a piece of bladder be tied tightly over the orifice, as the exhaustion proceeds the bladder will be pressed inwards, and will finally burst with a loud report. The pressure of the air is also proved by the experiment of the hemispheres, described in the article AIR.

soon as the exhaustion begins. A shrivelled apple will be restored to apparent freshness by the expansion of the air which it contains, but will resume its original appearance when the air is allowed to return. 4. The elasticity of air may be shown by placing a bladder under the receiver, not distended, and the mouth of which is tied up. On exhausting the receiver, the air contained in the bladder will expand it more and more, as more of the pressure from the exterior is removed; and the bladder will finally burst from the interior pressure. If a hole be made in the smaller end of an egg, and the egg be then placed in a wine-glass with this end down under the receiver, the small bubble of air which is always found in the larger end, will, by its expansion, force out the contents of the egg. On re-admitting the air, the contents of the egg will be forced back into the shell.

The first vacuum was made by Torricelli [TORRICELLI, in BIOG. Div.; BAROMETER], but the first air-pump was constructed by Otto von Guericke, who exhibited it publicly at the Imperial Diet of Ratisbon in 1654. It was an exhausting syringe, attached underneath to a spherical glass receiver, and worked somewhat like a common pump. The syringe was entirely immersed in water to render it air-tight. Shortly afterwards, Boyle constructed an air-pump in which the syringe was so far improved that the water could be dispensed with. He also first applied rack-work to the syringe. The second syringe and the barometer-gauge were afterwards added by Hawksbee, and several minor improvements were made by Gravesande and Smeaton. All the alterations which have been made since the time of the invention, however important, relate to the mechanism only, and not to the principle on which the pump acts.

In the Great Exhibition of 1851, among the instruments exhibited in Class X., the best air-pump was by Mr. Newman. It has a groundglass plate, and is furnished with two pumps with metal valves, on one of which are two barrels, open at the top. By this arrangement the receiver may be quickly exhausted to 0.4 inch or 0.5 inch. The other pump has a single barrel, with an oil-cistern at the upper part, and the air is lifted through a valve at the bottom of this cistern. "If anything re-enters the barrel it can only be oil, which is brought out with the air at the next up-stroke of the piston. The piston has a metal valve; but the opening of this valve is not necessary to the continuation of the exhaustion, as the piston at its lowest point passes below the aperture leading to the receiver. This construction of air-pump exhausts more thoroughly than any yet known." (Jury Report.') In some experiments which were tried with this pump, the reading of the barometer at the time was 30.08 inches, while the gauge of the pump stood at 30-06 inches.

A pump exhibited by Messrs. Watkins and Hill, on a plan suggested by Mr. Grove, has oil-silk valves, and is so constructed as to leave the least possible residue of air in the valve after each stroke of the piston. "The piston is solid, without a valve, and the shape of its lower part is an obtuse cone. Part of this cone rises at the top of each stroke above the aperture leading to the receiver; and the air which has entered the barrel is, by the down-stroke, forced through a valve at the apex of the hollow cone terminating the lower end of the barrel, to which the lower end of the piston fits very accurately. The pistonrods pass through air-tight leather collars in the tops of the barrels. This pump exhausted the air till the elastic force was only 0.05 inch of mercury."

Mr. Siemen's air-pump was exhibited by Knight and Sons. "It. consists of two cylinders of different diameters, the smaller one placed

below the larger, and separated from it by a plate forming the bottom of the upper and the top of the lower cylinder. A piston-rod, common to both cylinders, passes through a stuffing-box in the plate, attached to which are two valved pistons, working in their respective cylinders. The advantage of this construction is, that the pressure of the external air on the oiled silk valve of the larger cylinder is taken off by the vacuum formed in the smaller one, and in consequence no greater resistance is offered by the valve than that arising from its adhesion and tension. The exhaustion of this pump is very rapid, and in the trial amounted to 0.24 inch of mercury." The motion of the pistons is effected by means of a short crank with a jointed connecting-rod, converting the circular motion given by the lever handle into a vertical one, which is maintained by means of a cross-head with rollers working between guides. This pump is more particularly described in Mr. Tomlinson's 'Rudimentary Treatise on Pneumatics.'

In the air-pump exhibited by Varley and Sons a new construction is adopted. "It is worked by a continuous rotatory motion of the handle, slide-valves being used to open and close the communication. On the piston arriving at one end to expel air from the barrel, it is followed by rarefied air from the receiver; the slide-valve closes upon the receiver and connects the two sides of the piston; the residual air expands into the larger space, becomes equally rarefied, and the subsequent motion of the valve separates these spaces, and connects the receiver with the closed end. The piston then returns to exhaust air into this end of the barrel and to expel it from the other, and thus continuous exhaustion is kept up; for, how rare soever the air becomes, it keeps flowing after the piston continually. The barrel is twice filled for every entire revolution of the handle. This pump has a single barrel with double action; it exhausts quickly, and the exhaustion was found to be 0.05 inch for a moment, but could not be maintained."

In the French department, the double-barrelled air-pump exhibited by M. Breton had, instead of valves, a glass plate sliding over apertures communicating with the receiver, and the pumps and the motion of this glass plate is produced by the mechanism which works the pump. "The approximate exhaustion is first made by the ordinary alternate action of the barrels. The system of communication is then changed by shifting round the glass plate, which serves as a valve during onefourth of a revolution, when the rarefied air is condensed in one barrel and sucked into the other, whence it is ultimately ejected through a valve of oiled silk very close to the piston." In the air-pump exhibited by M. Deleuil, the barrels were of glass, and the valves, after M. Babinet's plan, were opened by means of wires passing through the pistons. The opening of the valves is thus rendered independent of the elastic force of the air left in the receiver, and the degree of exhaustion must depend on the air remaining after the action of the piston.

AISLE, or AILE (in Architecture), indirectly from the Latin word ala, a wing, through the French aile, which has the same signification. In French, this term is applied to the outlying and returning ends of a building, which we distinguish by its English equivalent, wing; such are the columned ends of the front of the British Museum. We apply the term aisle to the lateral divisions or passages of the interior of a church -those parts which lie between the flank walls and the piers, pillars, or columns, which flank the nave, or grand central division-when the structure is so arranged. Sometimes, but incorrectly, with reference to modern churches and chapels in this country, the mere passages or corridors which run between, and give access to the pews, are called aisles. Still more incorrectly, some writers, and even ecclesiastical writers, have called all the longitudinal divisions of the body of a church, aisles, thus including the nave under a designation which belongs only to its adjuncts and accessories.

The division of a church into what we term nave and aisles arose simply out of the difficulty which existed of spanning a great breadth with a roof without some intermediate support; and thus the greater Constantinian churches or basilicas of Rome were built with four rows of columns, forming five longitudinal divisions; that is, with two aisles on each side of the nave. This was imitated in subsequent structures, and the metropolitan churches of Milan and Paris were built in five divisions, or with four aisles, as they exist at the present time. That the custom of arranging the interiors of churches with aisles was continued in deference to ecclesiastical precedent, or at any rate long after the necessity for using the props which form them ceased architecturally, may be rendered clear by reference to the following fact. Most of our cathedrals and greater churches in this country are of later date than the roof of Westminster Hall, which, without intermediate support, spans a greater breadth than most of them can boast of; and yet they are, as a general rule, all divided into nave and aisles. In recent Gothic churches it is needless to add they are continued, even when, as in some of very small size, they are at once obstructive and costly.

In some English books, though perhaps in none of the present century, this term will be found written without the a-isle.

ALANINE (CH.NO.). When aldehyd-ammonia is acted on by hydrocyanic acid, and an excess of hydrochloric acid, a crystalline body, soluble in water, is formed, to which Strecker has given the name alanine. It crystallises in groups of prisms, which are insoluble in ether, nearly so in alcohol. Its solution has no effect upon test-papers. It sublimes at about 400°. It is homologous with glycocoll and leucine,

ALARUM. It is curious to mark how much ingenuity has been displayed within the last few years in the invention of alarums, and how many patents have been taken out for the inventions. In most of these contrivances there is some little bit of mechanism which gives a shrill sound whenever attention is required to be directed to any subject with which the alarum is associated.

Without describing any of the older forms, we may glance at a few of the modern suggestions. The 'travellers' alarum' is a small brass box about 2 inches in diameter by 14 thick. It has hands which may be set to any hour to awaken a sleeper. Mr. Allen, in 1844, registered an alarum intended to prevent injury to boilers from the water falling below its proper level; it consists of a float within the boiler, a steam-whistle on the exterior, and a tube of connection; when the water is at a proper height in the boiler, the float is buoyed up, and the whistle is silent; but when the water, and with it the float, descend too low, a little valve in the tube opens, and a current of steam from the boiler ascends to the whistle, which immediately gives forth a shrill sound, thereby indicating that the water has sunk too low in the boiler.

An alarum patented by Mr. Doull, is a 'railway whistle,' so constructed as to yield several notes, capable of being combined into a code of signals. A chemical alarum' by Mr. Mowbray consists of a copper cylinder, with a whistle at the top; a piece of carbonate of lime and a little muriatic acid are put into the cylinder, by which carbonic acid gas is speedily generated; and this is forced by some kind of mechanism into the whistle, whenever a sound is required to be produced. A contrivance by Mr. Hoare, described before the Society of Arts, consists of a chain of rods extending from end to end of a railway train, and moving freely on joints; at the end of the chain, in the guard's carriage, is a crank which, when the rods rotate on their axes, comes in contact with a hammer, and causes it to strike a bell; the driver, or the passengers in any carriage, can give a slight rotatory motion to the rods, and thus signals may be communicated. Up to the present time, however, all kinds of railway train signals have been sadly neglected. See further on this subject under RAILWAY.

But the busiest contriver of alarums, perhaps, is Mr. Rutter, who has called to his aid the marvels of electricity. In a patent for several such contrivances, taken out by him in 1847, one variety is the Fire Alarm', a complicated apparatus intended for use in large buildings. A galvanic battery is placed in one room, the alarum in another, thermometers in every room, and copper wires to connect all these pieces of apparatus. If the temperature of any room be greatly raised, as by accidental fire the rising mercury in the thermometer comes in contact with a metallic wire, which sets the galvanic battery in action, and this again works the alarum-bell in the same way as an electro-telegraphic clock, but with an adjustment intended to show in which room the rise of temperature has occurred. A second variety, the Trespass Alarum,' depends for its action on the placing, near every door and window, of a tube containing mercury, open at the top; the opening or closing of the door or window brings a small wire into contact with the surface of the mercury, and this completes a galvanic connection with a battery in another room: all the parts of the apparatus may be the same as those in the fire alarum, except by having open tubes of mercury near doors and windows, instead of thermometers in each room. A third variety, the 'Railway Alarum,' is intended to establish signals of communication between the guard and the engine-driver of a railway train. There is a copper wire carried through or upon or beneath each carriage, and connected with another in the adjoining carriage by a flexible metallic cord: the wire and cord being coated with gutta percha to secure isolation. There is thus a wire communication from end to end of the train. The guard has in his box or seat, a very small galvanic battery; and the engine-driver has a series of small studs connected with the rail on which his hand is usually resting. When the guard wishes to communicate with the engine-driver, he sends a slight galvanic shock through the wire to the spot on which the hand of the driver rests; and the duplication or variation of the shock may be made to indicate various signals.-It must be evident that great completeness and exactness would be necessary to render any of the above three kinds of alarum efficient for the purpose intended; and it may be added that, as yet, the contrivances have not come into actual use.

A floating alarum was suggested a few years ago by Mr Hobbs, of Bristol, to be moored to a sunken rock or other dangerous place at sea. The centre of the machine is an air-vessel or buoy. At each end is a box in which a whistle is fixed, whose mouth is protected from the water. As the water of the sea circulates in certain parts of the interior of the machine, it drives the air alternately from one end to the other, and impels it through the whistles; and the more violently the sea rocks the floating machine, the louder will the whistles give forth their sound. The proposal of the inventor is to make the buoy and whistles of such dimensions that the sound may be heard some miles distant. [Buoys.]