active pressure it sustains. In the atmospheric railway the piston moves just as fast as the train; and consequently to obtain an increased velocity, the load must be lightened in a more than corresponding ratio. But in the locomotive engines, the pistons, with a stroke varying perhaps from sixteen to twenty-four inches, act upon driving wheels of six or eight feet diameter, and will, therefore, recede before the impact of the steam with only one ninth or one sixteenth the velocity of the train. A far larger proportion of the force exerted by the elastic fluid is thus rendered available. Now that the experiment lately carrying on in Devonshire seems finally abandoned, the great atmospheric railway question,' may be regarded as settled. We only instance it, as a fair example of the fact already referred to, that it is their relative solution, with which problems involving difficulties of construction are chiefly concerned. For of the mechanical possibility of the machine there never was a doubt. With a certain area of exhausted tube, and a certain power working air pumps not placed too far apart, all the ordinary necessities of locomotion could be fully satisfied. And if we had known no other means of conveying trains at fifty miles an hour, this would have been sufficient. But the question was not only one of mechanical limit-it put in issue the comparative advantages of rival systems. The atmospheric tube must work better-that is, more cheaply and more usefully than the locomotive engine, to entitle it to supersede the latter in the public service.

On computing the relative limits of power in the locomotive engine, with reference to the three objects of economy, velocity, and safety, we discover that it is not the consideration of cost, nor the practical difficulties of construction, but the ne

* Our calculations, given above, appear to be fully borne out by the facts disclosed at the recent meeting of the South Devon Railway Company. It then transpired, that although upon the evidence given before Lord Howick's committee in 1845, the anticipated cost of the atmospheric tube had been estimated at 4 or 5000l. per mile, the expense really incurred was 11,1387. The working charges also were reckoned as certain to be far below those of the locomotives. By the test of some months' trial, over 35 miles of road, before the system was discarded, the relative cost appeared to be-locomotives, 2s. 6d., atmospheric 3s. 14d. per mile. The chairman, however, stated that by means of various improvements and items of economy, the expenses of the tube might be reduced to 3d. per mile below those of the locomotives. But even upon this estimate it would require a traffic of 90 trains per diem, or nearly one every quarter of an hour, running day and night, to pay 4 per cent. upon the additional outlay.

cessity of safety alone, which has assigned to our working velocities their present limits. So long as the chances of collision remain at their existing average, we cannot in prudence increase the rapidity; for even if we could construct our dead mechanism of strength sufficient to endure the concussion, the human machine will not bear it uninjured. Already, fatal results have supervened from accidents of that description, occasioned not by the effect of external injury, but simply from some internal disorganisation or shock to the system, produced by the sudden stoppage of rapid motion. But supposing that by better arrangements and more careful watching without resorting to the extreme measure of hanging a director or two we could reduce the danger of collision to the condition of a remote contingency, there are dangers and causes of disorder in the engine itself, and arising during the ordinary course of work, which must be taken into account. In a Report presented during 1846 to the French Minister of Public Works by M. de Boureuille, the chief of the railway department, and who had been commissioned to inquire into the means of ensuring safety in railway transit, we find the sources of danger thus indicated:

- even

'On analysing the strain upon the axles it was found to con'sist; first, of a vertical strain due either to the portion of 'the weight of the engine bearing upon that point, in consequence of the position of the centre of gravity, or to the action of the springs of the hinder axles in the six-wheeled engines. This strain being thus defined, even supposing that the parts upon which it acts are as near as possible to the point d'appui 'formed by the wheels, it tends nevertheless to bend the axle in a vertical direction. Secondly, a tension arising from the conoidal form of the peripheries of the wheels, and inequalities in the inclination of the rails: from which it happens that the 'peripheries of two wheels fixed upon one axle never touch the rails at the same point at the same time, and consequently each ' of the wheels will slip alternately upon the rails. If the twist ' resulting therefrom is not too violent, it keeps all the molecules in a constant state of vibration. Thirdly,-shocks arising from ' inequalities in the road caused by the undulations of the rails at 'their points of junction, on the passage of a train. These 'shocks increase in violence in proportion to the speed, and act in a direction at right angles to the axis of the axle. Fourthly, a strain of another description, arising from the oscillations 'of the carriages, acts upon the axles both in the direction of 'their length and at right angles thereto; increasing in force in 'proportion to the diameter of the wheels.'

Some of the dislocating forces here described increase as stated,

in direct proportion to the increase of velocity; others in a much higher ratio. The great cause of disturbance may be traced to the mode in which the expansive power of the steam is transmitted, through the axle, to the driving wheels, by means of a pair of piston-rods working upon cranks in the axle, and placed upon opposite sides of the line passing through the centre of gravity. Of necessity the two cranks cannot lie in the same plane, but must form a right angle with one another. Their forces, therefore, can never be in counterpoise. While the right-hand piston is at its dead point, the left-hand will be at a maximum; and while the axle is pushed forward on one side, it is pulled back on the other; and these interchanges of impulse, when at high speed, recur several times in every second. Enormous tendency to oscillation is thus produced, and the irregularity of motion, when once evolved, tends by the natural relation of the several parts and actions, to cause or to increase every other variety of eccentric force. The improvement, if such be possible, which should throw the axle of the driving wheels into revolution by some continuous and symmetrical impulse, will remove by far the largest part of the sources of danger and open wider limits to the possibility of greater speed.

In the process of weaving by the Power-loom we find an analogous example of velocity limited by the broken or alternating motion of the acting forces. The rapidity with which the shuttle can be thrown from side to side between the threads of the warp, is limited by the strength of the woof-thread it carries across. When the strain is so great as to cause more than a certain average number of breakings, the net product of the machine will be increased by working at a lower velocity. By a recent improvement, the shuttle is made at every vibration or shot,' to commence its motion slowly and increase in velocity as it proceeds; thus diminishing the strain upon the thread, and economising time, even in the four or six feet that constitute the average extent of each 'shot.' And by this means the looms are sometimes worked at a rate of 180 threads per minute, or 3 in every second. This will constitute the absolute limit of speed, under the existing form of construction. To extend it we must introduce a new principle, and discover some method of weaving the tissue in a cylindrical web; when the oscillation of the shuttle might be transformed into a continuous revolution, and the strain upon the woof, arising from the perpetual stoppage and change of motion, be annihilated.

The history of the first invention of the power-loom contains a curious proof, how much more difficult is the discovery of any

absolutely new principle, by which the old forms and processes of manipulation are entirely superseded, than the mere contrivance of means to imitate.by machinery what has been already done by hand. The latter requires only a very common endowment of the inventive faculty: the former demands the presence of creative genius. More than a hundred years before the invention of the steam loom, in the Philosophical Transactions for August 1678, there was given some account of a new engine to 'make woollen cloths without the help of an artificer,'-being a communication from a M. de Gennes, an officer belonging to the 'sea.' Much ingenuity is exhibited in the mechanical construction of this engine,' considering the time when it was produced: but in those days the only method of passing the woof-thread through the warp, was by the fingers of the weaver, assisted occasionally by a notched stick. And accordingly M. de Gennes, or whoever was the inventor of the machine, could hit upon no better plan than a complicated imitation of the human hand and arm, by which his shuttle is carried from side to side. Long afterwards, a common weaver invented the 'fly' shuttle, which is shot to and fro by springs; and modern inventors, having the benefit of this capital discovery, started from a high vantage ground, and have succeeded in bringing the power-loom to its present state of excellence.

But the difficulty with which a novel idea is caught or worked, is not the only one that stands in the way of the inventor. Improve our mechanism as we may, the human operator will always form an important element in our combinations; and will often prove by far the most intractable of our materials. Once let the workman be inured to the routine performance of duties on one machine, and it becomes a work of much time and cost to transfer him to another. The dearly acquired skill which constituted his chief capital is rendered useless; and the apprenticeship to his new tasks must be completed at much labour to himself and expense to his employers. We are assured by high authority that little short of a whole generation must expire, before the change can be thoroughly established. When some of the more remarkable inventions, like that of Arkwright's Spinning Jenny, were first introduced, it was found necessary to discard the whole of the trained operatives, and to intrust the attendance upon the new machines either to young children, or to recruits drawn from rustic neighbourhoods, who had never touched a spindle. It was no wonder that the skilled labourer' of the old system denounced and resisted the new; just as the old English archer resisted the introduction of the musket, after having acquired

by incessant practice from earliest childhood his unerring skill as a marksman, and so great muscular power that he could be recognised a mile off, merely from the size of his arms. The Spinning Jenny, indeed, presented such an enormous increase in speed and economy, that the old workers gave in without a struggle. But the weaving machines did not at first appear so hopelessly superior. The hand-loom weavers found themselves able to live in the race' with the steam engine, although at a terrible sacrifice. The competition has been persevered in, with melancholy pertinacity, to the present day; until Society has the burden and the scandal of a numerous class of individuals, industrious but ill-judging, who have, even in good times, to battle for a bare subsistence against fearful odds; and who, in the frequently recurring periods of depression, present the most afflicting spectacles.

The machine maker, in his turn, will endeavour to frustrate the innovations that tend to render his capital and experience, like the skill of the operative, in great measure valueless. If some new power should be discovered and trained to do for us more efficiently what steam does now, its adoption would be impeded by all the improvements in the steam engine, which four generations of engineers have combined to perfect. The most proper proportions of size and strength; the simplest arrangement of parts; the best form and construction of every valve and joint-even the machines that make the machines-have been long since ascertained and provided. The new power must be gifted with advantages very great and undeniable, if it can supersede, in all the rudeness of its primitive condition, the elaborate perfection of the established engines.

The common watch is in many of its parts a very ill-constructed machine. The train of wheelwork which transmits the motion of the mainspring, for example, is contrived on principles so faulty, that they would be scouted by every practised mechanician. Yet there can be no doubt that any attempt to introduce a better machine would utterly fail, as a commercial enterprise. Long used methods and ingenious engines have been specially provided to fashion and cut every one of the minuter parts which go to compose the existing instrument. Mr. Dent, in a lecture delivered at the Royal Institution, stated that every watch consisted of at least 202 pieces, employing probably 215 persons, distributed among 40 trades-to say nothing of the tool-makers for all of these. If we were now materially to alter the construction of the watch, all those trades would have to be relearnt, new tools and wheel-cutting engines to be devised; and the majority of the workmen to begin life