RAILWAYS AND STONE ROADS COMPARED.-MR. GURNEY'S STEAM ENGINE. -CONVENIENCE AND SAFETY OF STEAM CARRIAGES.-HANCOCK'S STEAM ENGINE.OGLE'S STEAM ENGINE. TREVITHECK'S INVENTION.-DR. CHURCH'S STEAM ENGINE.

(203.) WE have hitherto confined our observations on steampower, as a means of transport by land, to its application on railways. But modern speculation has not stopped there; various attempts have been made, and attended with more or less success, to work steam-carriages on common roads. The mere practicability of this project had long been regarded as very questionable; but enough has been done to show that the only doubt which can attend it, is as to whether it can be profitably resorted to, as a means of transport, and this question

has been materially affected by the recent extension of railways. In comparing the effect of a stone road with an iron railway, there are two circumstances which give great superiority and advantage to the latter: first, the resistance opposed by a railway to the moving power, no matter what that moving power may be, is considerably less in proportion to the load than on a stone road. The average resistance on a good level stone road, to the motion of carriages drawn at the speed usually attained by the application of horse-power, may be taken at about a thirty-sixth part of the load, while the resistance to a load drawn upon a railway at the same speed probably does not amount to a tenth part of this resistance. Thus the moving power, whatever it may be, would produce on a railway ten times the useful effect which it would produce on a stone road; secondly, the resistance which is opposed to the moving power on a level railway is much more uniform than on a stone road, and, consequently, the moving power is less subjected to jerks and inequalities. This renders the application of inanimate power more easy on the railway. Those inequalities of surface which increase the amount of resistance on stone roads as compared with railways also produce a jolting motion in the carriage, to counteract which, the use of springs become necessary. These springs render the motion of that part of the carriage which rests upon them different from that part of the carriage which supports them; and in the application of steam-machinery it becomes necessary so to connect the moving power with the wheels that the machinery may have one motion, and the wheels which are put in mechanical connexion with that machinery, and driven by it, shall have another motion. This, it is true, is the case with locomotive engines on railways; but owing to the greater smoothness and equality of the railway surface the difference between the motion of the carriage body suspended on springs and that of the wheels is much less than it would be on a stone road.

But besides the greater smoothness of railways compared with stone roads, the latter have another disadvantage, the effects of which have probably been exaggerated by those who are opposed to this application of steam-power. One of the

laws of adhesion long since developed by experiment, and established as a principle of practical science, is that the adhesion is greater between surfaces of the same than between surfaces of a different kind. Thus between two metals of the same kind, the adhesion corresponding to any given pressure is greater than between two metals of different kinds; between two metals of any sort the adhesion is greater than between metal and stone, or between metal and wood. Hence, the wheels of steam-carriages running on a railroad have a greater adhesion with the road, and therefore offer a greater resistance to slip round without the advance of the carriage, than wheels would offer on a turnpike road; for on a railroad the iron tire of the wheel rests in contact with the iron rail, while on a common road the iron tire rests in contact with the surface of stone, or whatever material the road may be composed of. Besides this, the dust and loose matter which necessarily collect on a common road, when pressed between the wheels and the solid base of the road, act somewhat in the manner of rollers, and give the wheels a greater facility to slip than if the road were swept clean, and the wheels rested in immediate contact with its hard surface. The truth of this observation is illustrated on the railroads themselves, where the adhesion is found to be diminished whenever the rails are covered with any extraneous matter, such as dust or moist clay. Although the adhesion of the wheels of a carriage with a common road, however, be less than those of the wheels of a steam-carriage with a railroad, yet still the actual adhesion on turnpike roads is greater in amount than has been generally supposed, and is quite sufficient to propel carriages drawing after them loads of large

amount.

The relative facility with which carriages are propelled on railroads and turnpike-roads equally affects any moving power, whether that of horses or steam engines; and whether loads be propelled by the one power or the other, the railroad, as compared with the turnpike-road, will always possess the same proportionate advantage; and a given amount of power, whether of the one kind or the other, will always perform a quantity of work less in the same proportion on a turn

pike-road than on a rail-road. But, on the other hand, the expense of original construction, and of maintaining the repairs of a rail-road, is to be placed against the certain facility which it offers to draught.

In the attempts which have been made to adapt locomotive engines to turnpike-roads, the projectors have aimed at the accomplishment of two objects: first, the construction of lighter and smaller engines; and, secondly, increased power. These ends, it is plain, can only be attained, with our present knowledge, by the production of steam of very high temperature and pressure, so that the smallest volume of steam shall produce the greatest possible mechanical effect. The methods of propelling the carriage have been in general similar to that used in the railroad engines, viz. either by cranks placed on the axles, the wheels being fixed upon the same axles, or by connecting the piston rods with the spokes of the wheels. In some carriages, the boiler and moving power, and the body of the carriage which bears the passengers, are placed on the same wheels. In others, the engine is placed on a separate carriage, and draws after it the carriage which transports the passengers, as is always the case on railways.

The chief difference between the steam engines used on railways, and those adapted to propel carriages on turnpike roads, is in the structure of the boiler. In the latter it is essential that, while the power remains undiminished, the boiler should be lighter and smaller. The accomplishment of this has been attempted by various contrivances for so distributing the water as to expose a considerable quantity of surface in contact with it to the action of the fire: spreading it in thin layers on flat plates; inserting it between plates of iron placed at a small distance asunder, the fire being admitted between the intermediate plates; dividing it into small tubes, round which the fire has play; introducing it between the surfaces of cylinders placed one within another, the fire being admitted between the alternate cylinders, — have all been resorted to by different projectors.

(204.) First and most prominent in the history of the application of steam to the propelling of carriages on turnpike roads stands the name of Mr. Goldsworthy Gurney, a medical gen

tleman, and scientific chemist, of Cornwall.

In 1822, Mr.

Gurney succeeded Dr. Thompson as lecturer on chemistry at the Surrey Institution; and, in consequence of the results of some experiments on heat, his attention was directed to the project of working steam-carriages on common roads; and he subsequently devoted his exertions in perfecting a steamengine capable of attaining the end he had in view.

The mistake which so long prevailed in the application of locomotives on railroads, and which, as we have shown, materially retarded the progress of that invention, was shared by Mr. Gurney. Without reducing the question to the test of experiment, he took for granted, in his first attempts, that the adhesion of the wheels with the road was too slight to propel the carriage. He was assured, he says, by eminent engineers, that this was a point settled by actual experiment. It is strange, however, that a person of his quickness and sagacity did not inquire after the particulars of these "actual experiments." So, however, it was; and, taking for granted the inability of the wheels to propel, he wasted much labour and skill in the contrivance of levers and propellers, which acted on the ground in a manner somewhat resembling the feet of horses, to drive the carriage forward. After various fruitless attempts of this kind, the experience acquired in the trials to which they gave rise at last forced the truth upon his notice, and he found that the adhesion of the wheels was not only sufficient to propel the carriage heavily laden on level roads, but was capable of causing it to ascend all the hills which occur on ordinary turnpike-roads. In this manner it ascended all the hills between London and Barnet, London and Stanmore, Stanmore Hill, Brockley Hill, and mounted Old Highgate Hill, the last at one point rising one foot in nine.

The boiler of Mr. Gurney's engine is so constructed, that there is no part of it in which metal exposed to the action of the fire is out of contact with water. If it be considered how rapidly the action of an intense furnace destroys metal when water is not present to prevent the heat from accumulating, the advantage of this circumstance will be appreciated. In the boiler of Mr. Gurney, the grate-bars