quate for obtaining the end proposed, since the smallest rut intercepts the lateral flow of the water; and, that the barrel shape confines carriages to the middle of the way, and thereby occasions these very ruts, roads are now laid flat, carriages drive indifferently over every part, the wear is uniform, and not even the appearance of a longitudinal furrow is to be seen. It may, therefore, confidently be hoped that wheels approaching to the cylindrical form will soon find their way into general use. The line of traction is mechanically best disposed when it lies exactly parallel to the direction of motion, and its power is diminished at any inclination of that line in the proportions of the cosine of the angle to radius. When obstacles frequently occur, it had better perhaps receive a small inclination upwards, for the purpose of acting with most advantage when those are to be overcome. But it is probable that different animals exert their strength most advantageously in different directions, and therefore practice alone can determine what precise inclination of this line is best adapted to horses, and what to oxen. These considerations are, however, only applicable to cattle drawing immediately at the carriage; and the convenience of this draft as connected with the insertion of the line of traction, which continued ought to pass through the axis of the wheels, introduces another limit to their size. Springs were in all likelihood applied first to carriages, with no other view than to accommodate travellers. They have since been found to answer several important ends. They convert all percussion into mere increase of pressurethat is, the collision of two hard bodies is changed by the interposition of one that is elastic, into mere accession of weight. Thus the carriage is preserved from injury, and the materials of the road are not broken: and, in surmounting obstacles, instead of the whole carriage with its load being lifted over, the springs allow the wheels to rise, while the weight suspended upon them is scarcely moved from its horizontal level. So that, if the whole of the weight could be supported on the springs, and all the other parts supposed to be devoid of inertia, while the springs themselves were very long and extremely flexible, this consequence would clearly follow, however much it may wear the appearance of paradox; that such a carriage may be drawn over a road abounding in small obstacles without agitation, and without any material addition being made to the moving power or draft. It seems, therefore, probable that, under certain modifications of form and material, springs may be applied with advantage to the very heaviest wagons; and consequently, if any fiscal regulations exist either in regard to the public revenue or to local taxation, tending to discourage the use of springs, they should forthwith be removed. Although the smoothness of roads and the application of springs are beneficial to all carriages and to all rates of travelling, yet they are eminently so in cases of swift conveyance, since obstacles when springs are not interposed, require an additional force to surmount them beyond the regular draft, equal to the weight of the load multiplied by the sine of the angle intercepted on the periphery of the wheel between the points in contact with the ground and with the obstacle, and therefore proportionate to the square of its height; and a still further force, many times greater than the former when the velocity is considerable, to overcome the inertia, and this increases with the height of the obstacle, and with the rapidity of the motion, both squared. But, when springs are used, this latter part, by far the most important, almost entirely disappears, and their beneficial effects in obviating the injuries of percussion are proportionate also to the velocities squared. The advantages consequent to the draft from suspending heavy baggage on the springs, were first generally perceived about 40 years since on the introduction of mail coaches; then baskets, and boots were removed, and their contents were heaped on the top of the carriage. The accidental circumstance, however, of the height being thus placed at a considerable elevation, gave occasion to a prejudice, the cause of innumerable accidents, and which has not, up to the present time, entirely lost its influence; yet a moment's consideration must be sufficient to convince any one, that when the body of a carriage is attached to certain given points, no other effect can possibly be produced by raising or by depressing the weights within it, than to create a greater or a less tendency to overturn. The extensive use of wagons suspended on springs for conveying heavy articles, introduced within these two or three last years, will form an epoch in the history of internal land communication, not much inferior perhaps in importance to that when mail coaches were first adopted, and the extension of vans in so short a time to places the most remote from the metropolis, induces a hope and expectation, that as roads improve, the means of preserving them will improve also, possibly in an equal degree, so that permanence and consequent cheapness, in addition to facility of conveyance, will be distinguished features of the M'Adam system. FROM THE MECHANIC'S MAGAZINE. RELATIVE EFFECTS OF STEAM AND GUNPOWDER. A SERIES of well conducted and decisive experiments were made by Count Rumford, on the expansive force of fired gunpowder, a detailed account of which, together with engravings of the apparatus employed in so hazardous an undertaking, are to be found in the Philosophical Transactions for 1797; of the facts and the truth of the results, no shadow of doubt can exist, and they therefore deserve serious attention. In these experiments the Count put the small quantity of twelve grains of gunpowder into an iron chamber, of which the bore was a quarter of an inch; weights were placed upon a valve closing the orifice; the powder was fired, and it was found to exert a force of 9431 atmospheres. Seventeen grains, when fired in a similar bore, could not raise a weight of 8081 lbs. placed on a valve which had an area of the twentieth part of a square inch; but eighteen grains raised that weight, and thus exerted a force equal to 10,977 atmospheres, or 165,000 lbs. on each square inch. In these experiments the powder filled only about half the cavity; hence it expanded to double its bulk, and still exerted this amazing force; but when the whole cavity, equal only to one tenth of a square inch, was filled with twenty six grains of powder (a quantity insufficient to charge a pocket pistol) the solid cylinder of hammered iron was burst asunder, though it was in every part an inch and a quarter thick, or five times the bore; to effect which would require a force equal to 54,750 atmospheres, or 410,624 lbs. on the square inch. This latter result rests upon a calculation of the force requisite to burst an iron cylinder of the given dimensions. I have examined that calculation, and believe it to be correct; but, whatever doubt may be entertained as to this fact, there can be none with regard to the former-for, in them, the power was estimated by the dead weight which the fired gunpowder actually lifted when placed over an orifice of a given size. In these we find a valve, the surface of which is but the twentieth part of a square inch, loaded with nearly four tons, and the elastic force overcomes and lifts it-a fact almost incredible; and, compared with which, the load on the valve of the highest pressure steam generator that ever was invented sinks into absolute insignificance. It has been stated that from seven hundred to a thousand pounds per square inch is the elasticity of the steam used by Mr. Perkins in discharging pistol bullets. These are fired, in rapid succession, from a single gun barrel against a metal target, one hundred feet distant, and they are completely flattened on it; this, however, is not equal to the effect of a horse pistol, and it is much inferior to that produced by the discharge of a musket. The rapidity of the discharge is, however, to be considered; but, if we reflect that coals, water, pipes, a generator, and furnace, are all requisite to keep up the necessary supply of steam, we shall find that as much bulk and weight are attached to a single gun barrel as would equal that of a field piece, which, in my opinion, would discharge more grape, and with infinitely greater force in a given time, than the whole of Mr. Perkins' apparatus. The elastic force, indeed, of steam may be increased by an increased heat, and hence a greater power will be obtained; but, to effect this, there must be a proportionable increase of the size of the pipes, generator, furnace, &c. and this in a much greater degree than is usually supposed even by practical men. It is also to be kept in mind, that there is a limit to the expansibility of steam, which the nature of materials will not allow us to go beyond; and we may presume that no practical use will ever be made of steam beyond one or two thousand pounds on the square inch. Indeed, when we reflect that, at a temperature of seven hundred degrees Fahrenheit, steam only occupies four times the space of water, whilst, at a temperature of two hundred twelve degrees, it expands 18,000 times, we will discover another difficulty in the fact, that one quarter of the power generated would be required to force the water into the generator, which must be proportionably large to raise so much water to so high a temperature, and keep up that constant stream of it which is absolutely necessary for sustaining a continuous discharge of balls even from so small a bore as that of a gun barrel. What then will we think of an attempt to apply steam to the discharge of ordnance? This, indeed, requires no serious attention, and it is my opinion, after a careful examination of the subject, that a twenty four pounder steam cannon, equally effective with our common guns of the same calibre never will be constructed the thing is manifestly impossible. POETRY. ORIGINAL. TO A FRIEND. WE'VE met again,-and life is still, Though lovely was that summer hour, Though deepest in our bosoms shrin'd, Though on our life's bright morning sky, The clouds of sorrow have been seen,- We've shed on pale cold brows the tear, Why should the heart of man be sad? The starry sky is bright and blue, Its smile of love on Earth bestowing, And all that meets the admiring view With beauty, light and life is glowing. Then let us pledge those happy hours, And call their sweetest moments back, When first we stray'd through Learning's bowers, And gaz'd on Science's radiant track. Oh! then we listen'd to the lyre Of some high bard, whose stirring strains Awoke within our breasts the fire That kindled in his glowing veins. |