the experiments of Mr. Vince) be no acceleration of motion on the principle of falling bodies.

However, though a carriage cannot, as we think we have shewn, be moved ten miles in one hour, with a similar expenditure of power than in two, it is very interesting to know that it can be moved with the same expenditure, (excepting the resistance of the air.) In many cases dispatch is of so much consequence, that the elucidation and application of this rule will probably lead to very important results. Many persons, however, are very sceptical on this subject, and contend that the experiments of Vince and Coulomb do not authorise any such conclusions as have been drawn from them. It has been asked, if the same constant force will move a carriage as well at a high as at a low velocity, why we do not see something like this in practice; why a carriage moved by a steam-engine instead of acquiring, as it proceeds, a high degree of velocity, moves on at one uniform rate after it has overcome the vis inertiæ at the commencement of its journey? We think the reason is very obvious. A locomotive steam-engine does not exert the same constant force on the peripheries of the wheels of the carriage, when it moves at different velocities. For instance, suppose the piston of an engine to move 220 feet in a minute, and to impel the peripheries of the travelling wheels at a velocity of two miles, and with a force just sufficient to overcome the friction, how can the speed be augmented without increasing the power of the engine? If the diameter of the wheels be increased with the view of increasing the speed, the force with which they are impelled will be diminished in the same proportion; and the engine will stop, unless the pressure is increased. To increase that, of course, will be to augment the power. As it is obvious, therefore, that a steam-engine cannot exert the same force at different velocities, some other means must be devised for putting to the test of experiment the rule laid down in the Scotsman.

We now come to the most important and interesting part of this article. As none of the experiments of Vince or Coulomb (so far as we have seen or heard them detailed) were made with bodies resembling railway waggons, either in form, or in the nature of their motion, the correctness of the conclusions deduced from them with respect to such carriages, was doubted by many persons of considerable scientific attainments. It became desirable, therefore, that other experiments should be tried, with carriages

upon railways, which, of course, would be much more satisfactory. This, however, it did not, at first sight, appear very easy to accomplish in a satisfactory manner: but Mr. Roberts, of this town, recently devised a mode of determining the point, which appears to us wholly unobjectionable, and which exhibits, in a high degree, the simplicity and facility of execution, by which that gentleman's inventions are so eminently distinguished. It was very

difficult to devise means for measuring accurately the frietion of a carriage moving over a railway; but it occurred to Mr. Roberts, that the difficulty would be obviated if the railway were made to move under the carriage. When this idea once presented itself, it was easy to reduce it to prac tice. Mr. Roberts therefore constructed an apparatus, of which fig. 654 will give a pretty correct notion.

A is a small waggon with four cast iron wheels, placed on the peri phery of a cast iron drum B, three feet in diameter, and six inches broal, (which acts as the rail-road.) This drum is fastened on the same shaft a the pulley C, which is driven at different speeds by a strap from another pulley. The waggon is attached by a wire to one of Marriot's patent weighing machines D, for the purpose of measuring the friction, and the board G, prevents the current of air, occasioned by the motion of the drum, from acting upon the carriage. Now if the drum be driven with any given velocity, say four miles an hour, in the direction indicated by the mark E, and the waggon held in its place by the wire which attaches it to the index, it is perfectly obvious that the wheels will revolve on the drum in precisely the same manner as if the waggon moved forward on a horizontal road; and the friction will also be the same, except, perhaps, a small addition occasioned by the curvature of the drum, but which will not affect the relative frictions of different speeds. As the waggon is sta tionary, the resistance of the air will be entirely got rid of; and the index of the machine will indicate the precise amount of traction necessary to overcome the friction. Of course, in making the experiment, it will be necessary to keep the centre of the waggon exactly over the axis of the drum; for if it were permitted to go beyond the centre, a part of the weight would be added to the friction; if, on the contrary, it was brought nearer the index, a part of the weight would act against the friction, and diminish the apparent quantity. The tempering screw F, is therefore added to keep the waggon in its proper situation, in whatever way the spring of the weighing machine may be acted upon by the friction.

This simple apparatus having been constructed, a number of experi ments were made, chiefly with a view to determine whether the friction were the same at different velocities. The waggon was loaded with fifty pounds, (including its own weight) and the drum was driven at different velocities, varying from two to twenty-four miles an hour on the periphe ry; but in every case, the friction, as indicated by the weighing machine, was precisely the same. No increase of speed affected the index at all. but on increasing the weight, it immediately shewed a corresponding increase of friction.

of the fact, that the friction on a railway is the same for all velocities; and that a carriage may be propelled twenty miles in one hour, with the same amount of force which would be necessary to drive it twenty miles in ten hours, provided the resistance of the atmosphere was out of the question: and, if the carriage was properly constructed, that would not amount to much. In other words, goods may be conveyed from Manchester to Liverpool, on a rail-road, with very nearly the same expenditure of steam, whether they are carried two miles, or four miles, or twenty miles an hour. A steam engine, which will propel twenty tons at four miles an hour, will, with the same expense of coals, propel ten tons at eight miles an hour; so that, with the smaller load, it might make a journey to Liverpool and back, in the same time which would be occupied in going thither with the larger load Or, to put the matter in another shape: suppose a four-horse engine will convey forty tons to Liverpool in eight hours, an eight horse engine will convey the same weight thither in four hours. There will be the same expenditure of steam in both cases, but, in the latter, a saving of half the time; a saving which, we need not add, will frequently be of immense importance."

These practical results are very satisfactory, as the hope of propelling carriages at a suitable speed, for the more rapid dispatch of business, and conveyance of passengers, is thereby placed almost beyond a doubt.

We ought to notice here, the striking difference in the force requisite to give rapid motion on a rail-road to that on a canal or navigable river. These latter are governed by a totally different law, as the resistance, or head of water on the bows of the boat, increase as the squares of its velocity; consequently it will require four times the power to double the speed. But, on the other hand, it must be admitted, that in all speeds under three miles per hour, the canal has a decided advantage, as the force increases as the speed diminishes.

With respect to the horse, it is well known, that his power decreases as his speed increases; and that when he is travelling at his greatest speed, which, with a weight, seldom exceeds 13 miles per hour, he is able to exert little or no strength. We, therefore, take it for granted, that in the present improved state of our manufactures, artificial power of some description must be resorted to, and whatever experience may prove to be the most economical, the application of that power is the most important part of the