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all times nearly the same amount of weight to bear, and consequently the entire weight was at all times pretty equally distributed amongst the four wheels of the locomotive. Thus the four floating pistons were ingeniously made to serve the purpose of springs in equalising the weight, and in softening the jerks of the machine; the weight of which, it must also be observed, had been increased, on a road originally calculated to bear a considerably lighter description of carriage. This mode of supporting the engine remained in use until the progress of spring-making had so far advanced that steel springs could be manufactured of sufficient strength to be used in locomotives. :

The result of the actual working of George Stephenson's new locomotive and improved road amply justified the promises held forth in his “specification.” The traffic was conducted with greater regularity and economy, and the superiority of the locomotive engine, as compared with horse traction, became more apparent. And it is a fact worthy of notice, that the identical engines constructed by Mr. Stephenson in 1816, are to this day to be seen in regular useful work upon the Killingworth railway, conveying heavy coal trains at the speed of between five and six miles an hour, probably as economically as any of the more perfect locomotives now in use.

Mr. Stephenson's endeavours having been attended with such marked success in the adaptation of locomotive power to railways, his attention was called, by many of his friends, about the year 1818, to the application of steam to travelling on common roads. It was from this point, indeed, that the locomotive had been started, Trevithick's first engine having been constructed with this special object. Stephenson's friends having observed how far behind he had left the original projector of the locomotive in its application to railroads, perhaps naturally inferred that he would be equally

CHAP. XII.] INVENTION OF THE DYNAMOMETER.

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successful in applying it to the purpose for which Trevithick and Vivian originally intended it.

But the accuracy with which he estimated the resistance to which loads were exposed on railways, arising from friction and gravity, led him at a very early stage to reject the idea of ever successfully applying steam power to common road travelling. In October 1818 he made a series of careful experiments, in conjunction with Mr. Nicholas Wood, on the resistance to which carriages were exposed on railways, testing the results by means of a dynamometer of his own construction. His readiness at all times with a contrivance to enable him to overcome a difficulty, and his fertility in expedients, were in no respect more strikingly displayed than in the invention of this dynamometer. Though it was found efficient for the purpose for which it was contrived, it will not of course bear a comparison with other instruments for a similar purpose that have since been invented. The series of practical observations made by means of this instrument were interesting, as the first systematic attempt to determine the precise amount of resistance to carriages moving along railways.* It was thus for the first time ascertained by experiment that the friction was a constant quantity at all velocities. Although this theory had long before been developed by Vince and Coulomb, and was well known to scientific men as an established truth, yet at the time when Mr. Stephenson made his experiments, the deductions of philosophers on the subject were neither believed in nor acted upon by practical engineers. And notwithstanding that the carefully conducted experiments in question went directly to corroborate the philosophical theories on the

. The experiments are set forth in detail in “A Practical Treatise on Railroads and Interior Communication in General.” By Nicholas Wood, Colliery Viewer, C.E. London : Hurst, Chance, and Co., ed. 1831, pp. 197

subject, it was a considerable time (so great were the prejudices then existing) before the conclusions which they established received the sanction of practical men.

It was maintained by many that the results of these experiments led to the greatest possible mechanical absurdities. For example, it was insisted that, if friction was constant at all velocities upon a level railway, when once a power was applied to a carriage, which exceeded the friction of that carriage by the smallest possible amount, such excess of power, however small, would be able to convey the carriage along a level railway at all conceivable velocities. When this position was taken by those who opposed the conclusions to which Mr. Stephenson had arrived, he felt the greatest hesitation in maintaining his own views; for it appeared to him at first sight really the absurdity which his opponents asserted it to be. Frequent and careful repetition of his experiments, however, left no doubt upon his mind as to the soundness of his conclusion,- that friction was uniform at all velocities. Notwithstanding the ridicule that was thrown upon his views by many persons with whom he associated at the time, he continued to hold to this conclusion as a fact positively established; and he soon afterwards boldly maintained, that that which was an apparent absurdity was indeed an inevitable consequence, and that every increase of speed involved a necessary expenditure of power almost in a direct ratio.

It is unnecessary at this time of day to point out how obvious this consequence is, and how it is limited and controlled by various circumstances; nevertheless it is undoubted, that could you always be applying a power proportionately in excess of the resistance, a constant increase of velocity would follow without any limit. This is so obvious to professional men now, and is indeed so axiomatic, that it is unnecessary further to illustrate the position; and the discussions which took place on the subject, when the results of Mr. Stephenson's

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experiments were announced, are only here alluded to for the purpose of showing the difficulties he had to contend with and overcome at the time, and how small was the amount of science then blended with engineering practice.

The other resistances to which carriages are exposed, were at the same time investigated by Mr. Stephenson. He perceived that these resistances were mainly three; the first being upon the axles of the carriage, the second (which may be called the rolling resistance) being between the circumference of the wheel and the surface of the rail, and the third being the resistance of gravity. The amount of friction and gravity was accurately ascertained; but the rolling resistance was a matter of greater difficulty, being subject to great variation. He however satisfied himself that it was so great when the surface presented to the wheel was of a rough character, that the idea of working steam carriages upon common roads was dismissed by him as entirely out of the question. Even so early as the period alluded to (1818) he brought his theoretical calculations to a practical test: he scattered sand upon the rails when an engine was running, and found that a small quantity was quite sufficient to retard and even to stop the most powerful locomotive that he had at that time made. And he never failed to urge this conclusive experiment upon the attention of those who were at that time wasting their money and ingenuity upon the vain attempt to apply steam power to the purpose of travelling on common roads.

Having ascertained that resistance might be taken as represented by 10 lbs. to a ton weight on a level railway, it became obvious to him that so small a rise as 1 in 100 would diminish the useful effort of a locomotive by upwards of 50 per cent. This was demonstrated by repeated experiments, and the important fact, thus rooted deeply in his mind, was It was owing in a great measure to these painstaking experiments that he thus early became convinced of the vital importance, in an economical point of view, of reducing the country through which a railway was intended to pass as nearly as possible to a level. Where. as in the first coal railways of Northumberland and Durham, the load was nearly all one way, – that is, from the colliery to the shippingplace, - it was an advantage to have an inclination in that direction. The strain on the powers of the locomotive was thus diminished, and it was an easy matter for it to haul the empty waggons back to the colliery up even a pretty steep incline. But when the loads were both ways, it appeared obvious to him that the railroad must be constructed as nearly as possible on a level. The strong and sagacious mind of Stephenson early recognised this broad principle; and he had so carefully worked out the important facts as to the resistance offered by adverse gradients, that he never swerved from it. At a much later period, when the days of “fast” engineering had arrived, while many thought him prejudiced on this point, he himself clung tenaciously to it, and invariably insisted upon the importance of flat gradients. It is true, great and important additions were made to the powers of the locomotive; but no sooner were these effected, than lines of steeper and still steeper gradients were devised, until, as he used to declare, engineers were constantly neutralising the increased powers of the engine, and in precisely the same degree diminishing the comparative advantages of railways over common roads.

These views, thus early entertained, originated, in Mr. Stephenson's mind, the peculiar character of railroad works as distinguished from all other roads; for, in railroads, he early contended that large sums would be wisely expended in perforating barriers of hills with long tunnels, and in raising the lower levels with the excess cut down from the adjacent

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