has been here described is capable of evaporating 77 cubic feet of water per hour, while the early locomotives could only evaporate 16 cubic feet per hour. This evaporation, however, is inferior to that which I have ascertained myself to be produced by engines in regular operation on some of the northern railways. In an experiment made in July, 1839, with the Hecla engine, I found that the evaporation in a trip of ninety-five miles, from Liverpool to Birmingham, was at the rate of 93.2 cubic feet per hour, and in returning the same distance it was at the rate of 85.7 cubic feet per hour, giving a mean of 89 cubic feet per hour nearly. The Hecla weighed 12 tons; and its dimensions and proportions corresponded very nearly with those of the engine above de

scribed.

In a course of experiments which I made upon the engines then in use on the Grand Junction Railway in the autumn of 1838 I found that the ordinary evaporating power of these engines varied from eighty to eighty-five cubic feet per hour.

Engines of much greater dimensions, and consequently of greater evaporating power, are used on the Great Western Railway. In the autumn of 1838 experiments were made upon these engines by Mr. Nicholas Wood and myself, when we found that the most powerful engine on that line, the North Star, drawing a load of 110 tons gross, engine and tender inclusive, at 30 miles an hour, evaporated 200 cubic feet of water per hour. The same engine drawing a load of 194 tons at 18 miles an hour evaporated 141 cubic feet per hour, and when drawing 45 tons at 38 miles an hour evaporated 198 cubic feet of water per hour.

It has been already shown that a cubic foot of water evaporated per hour produces a gross amount of mechanical force very little less than two-horse power, and consequently the gross amount of mechanical power evolved in these cases by the evaporation of the locomotive boilers will be very nearly twice as many horse-power as there are cubic feet of water evaporated per hour. Thus the evaporation of the Hecla, in the experiments made in July, 1839, gave a gross power of about one hundred and eighty horses, while the evaporation of the North Star gave a power of about four hundred horses. In stationary engines about half the gross

power evolved in the evaporation is allowed for waste, friction, and other sources of resistance not connected with the load. What quantity should be allowed for this in locomotive engines is not yet ascertained, and therefore it is impossible to state what proportion of the whole evaporation is to be taken as representing the useful horse-power.

(199.) The great uniformity of resistance produced by the traction of carriages upon a railway is such as to render the application of steam power to that purpose extremely advantageous. So far as this resistance depends on mechanical defects, it is probably rendered as uniform as is practicable, and in proportion to the quantity of load carried is reduced to as small an amount as it is likely to attain under any practicable circumstances. Until a recent period this resistance was ascribed altogether, or nearly so, to mechanical causes. The inequalities of the road-surface, the friction of the axles of the wheels in their bearings, and the various sources of resistance due to the machinery of the engine, being the principal of these resistances, were for the most part independent of the speed with which the train was moved; and it was accordingly assumed in all calculations respecting the power of locomotive engines that the resistance would be practically the same whatever might be the speed of the train. It had been well understood that so far as the atmosphere might offer resistance to the moving power this would be dependent on the speed, and would increase in a very high ratio with the speed; but it was considered that the part of the resistance due to this cause formed a fraction of the whole amount so insignificant that it might be fairly disregarded in practice, or considered as a part of the actual computed resistance taken at an average speed.

It has been, until a late period, accordingly assumed that the total amount of resistance to railway trains which the locomotive engines have had to overcome was about the two hundred and fiftieth part of the gross weight of the load drawn: some engineers estimated it at a two hundred and twentieth; others at a two hundred and fiftieth; others at a three hundred and thirtieth part of the load; and the two hundred and fiftieth part of the gross load drawn may perhaps be

considered as a mean between these much varying estimates. What the experiments were, if any, on which these rough estimates were based, has never appeared. Each engineer formed his own valuation of this effect, but none produced the experimental grounds of their opinion. It has been said that the trains run down the engine, or that the drawing chains connecting the engine slacken in descending an inclination of sixteen feet in a mile, or. Numerous experiments, however, made by myself, as well as the constant experience now daily obtained on railways, show that this is a fallacious opinion, except at velocities so low as are never practised on railways.

(200.) In the autumn of 1838 a course of experiments was commenced at the suggestion of some of the proprietors of the Great Western Railway Company, with a view to determine various points connected with the structure and the working of railways. A part of these experiments were intended to determine the mean amount of the resisting force opposed to the moving power, and this part was conducted by me. After having tried various expedients for determining the mean amount of resistance to the moving power, I found that no method gave satisfactory results except one founded on observing the motion of trains by gravity down steep inclined planes. When a train of waggons or coaches is placed upon an inclined plane so steep that it shall descend by its gravity without any moving power, its motion, when it proceeds from a state of rest, will be gradually accelerated, and if the resistance to that motion was, as it has been commonly supposed to be, uniform and independent of the speed, the descent would be uniformly accelerated: in other words, the increase of speed would be proportional to the time of the motion. Whatever velocity the train would gain in the first minute, it would acquire twice that velocity at the end of the second minute, three times that velocity at the end of the third minute, and so on; and this increase of velocity would continue to follow the same law, however extended the plane might be. That such would be the law which the descending motion of a train would follow had always been supposed, up to the time of the experiments now referred to; and it was even maintained by some that

such a law was in strict conformity with experiments made upon railways and duly reported. The first experiments instituted by me at the time just referred to afforded a complete refutation of this doctrine. It was found that the acceleration was not uniform, but that with every increase of speed the acceleration was lessened. Thus if a certain speed were gained by a train in one second when moving at five miles an hour, a much less speed was gained in one second when moving ten miles an hour, and a comparatively small speed was gained in the same time when moving at fifteen miles an hour, and so on. In fact, the augmentation of the rate of acceleration appeared to diminish in a very rapid proportion as the speed increased: this suggested to me the probability that a sufficiently great increase of speed would destroy all acceleration, and that the train would at length move at a uniform velocity. In effect, since the moving power which impels a train down an inclined plane of uniform inclination is that fraction of the gross weight of the train which acts in the direction of the plane, this moving power must be necessarily invariable; and as any acceleration which is produced must arise from the excess of this moving power over the resistance opposed to the motion of the train, from whatever causes that resistance may arise, whenever acceleration ceases, the moving force must necessarily be equal to the resistance; and therefore, when a train descends an inclined plane with a uniform velocity, the gross resistance to the motion of the train must be equal to the gross weight of the train resolved in the direction of the plane; or, in other words, it must be equal to that fraction of the whole weight of the train which is expressed by the inclination of the plane. Thus if it be supposed that the plane falls at the rate of one foot in one hundred, then the force impelling the train downwards will be equal to the hundredth part of the weight of the train. So long as the resistance to the motion of the train continues to be less than the hundredth part of its weight, so long will the motion of the train be accelerated; and the more the hundredth part of the weight exceeds the resistance, the more rapid will the accele

exceeds the resistance, the less rapid will the acceleration be. If it be true that the amount of resistance increases with the increase of speed, then a speed may at length be attained so great that the amount of resistance to the motion of the train will be equal to the hundredth part of the weight. When that happens, the moving power of a hundredth part of the weight of the train being exactly equal to the resistance to the motion, there is no excess of power to produce acceleration, and therefore the motion of the train will be uniform.

Founded on these principles, a vast number of experiments were made on planes of different inclinations, and with loads of various magnitudes; and it was found, in general, that when a train descended an inclined plane, the rate of acceleration gradually diminished, and at length became uniform; that the uniform speed thus attained depended on the weight, form, and magnitude of the train and the inclination of the plane; that the same train on different inclined planes attained different uniform speeds on the steeper planes a greater speed being attained. From such experiments it followed, contrary to all that had been previously supposed, that the amount of resistance to railway trains had a dependence on the speed; that this dependence was of great practical importance, the resistance being subject to very considerable variation at different speeds, and that this source of resistance arises from the atmosphere which the train encounters. This was rendered obvious by the different amount of resistance to the motion of a train of coaches and to that of a train of low waggons of equal weight.

The former editions of this work having been published before the discovery which has resulted from these experiments, the average amount of resistance to railway trains, there stated, and the conclusions deduced therefrom, were in conformity with what was then known. It was stated that the resistance to the moving power was practically independent of the speed, and on level rails was at the average rate of about seven pounds and a half per ton. This amount would be equivalent to the gravitation of a load down an inclined plane falling, and consequently in ascending such a plane the moving power would have to encounter twice