Newcomen, Watt, and others, is fairly due the merit of the invention of the steam engine, as the Marquis of Worcester's projects were wild and fanciful. There were many difficulties to overcome before the steam engine could be made applicable to purposes of locomotion. No doubt the double-acting steam engine had, in the year 1782, been brought by the genius of Watt to such a degree of perfection as to admit of few subsequent improvements; but how unsuitable such an engine must be, with its heavy frame work, huge fly wheel, its beam, condenser, and cumbrous boiler, for locomotive purposes. Had these clever adaptations for stationary steam engines not been got rid of, the locomotive engine would have made little advancement. Who can doubt that repeated trials and experiments must have been made ere the least excellence could have been attained in the application of the steam engine as the motar on railways.

It must have been obvious to engineers, almost from the first, that locomotive engines must be differently constructed from the common condensing engine. From the period of Watt till within the last 30 or 40 years high-pressure engines were rarely used in this country: indeed a strong prejudice existed against them from ap

prehension of their safety. These prejudices gradually wore out where small steam engines were used: and when steam power came into use

from 15 to 20 years ago - in Scotland for farm purposes, high pressure or noncondensing engines, on the reciprocating principle, were generally preferred, from cheapness, as the motive power for barn machinery. Engines of this description are now in general use at farms in Scotland and the borders of England.* The usual pressure at which these high-pressure engines are worked is from 30 to 35 lbs. on the square inch; and no accident with them has yet occurred at farms, showing that with good management there is perfect safety.

TREVITHICK AND VIVIAN'S LOCOMOTIVE

ENGINE.

Steam engines, however, modelled after the form of condensing engines, were obviously very unsuitable for portable or locomotive purposes. The first high-pressure engine, it is generally admitted, applicable for the latter purpose, was the invention of Richard Trevithick and Andrew

*The author has now in the press a work, for which a premium was awarded, on the application of the steam engine to farm purposes.

Vivian, engineers, of Camborne, in Cornwall. In the specification of the patent, dated March 24. 1802, it is described "for improving the construc"tion of steam engines and the application thereof "for driving carriages on rails, and turnpike roads, "and other purposes;" likewise, that their engine will produce a "more equable rotatory motion on "the several parts of the revolutions of any axis "which is moved by the steam engine, by causing "the piston rods of two cylinders to work on the "said axis by means of cranks at turn asunder."

It is also mentioned that they occasionally propose "to make the external periphery of the wheels of "carriages uneven by projecting heads of nails, or "bolts, or cross grooves, or fittings to rail-rods, "and that in cases of a hard pull to cause a lever "bolt or claw to project through the rims of one "or both wheels, so as to take hold of the ground; "but that in general the ordinary structure of the "external surface of the wheels would be suffi"cient; that the form of the engine might be "varied by changing the relative velocity of rota"tion of the wheels as compared with that of the "axis by shifting the gear, or by having toothed "wheels of different sizes; the body of the carriage "to be made of any convenient form according to "its use." In this clever invention the steam was proposed to be worked either at high pressure or not.

If the former, at a pressure from 60 to 80 lbs. on the square inch, and the boiler was made of a cylindrical form, to bear the expansive action of strong steam, having a bent tube like the letter U within it, to increase the heating surface. The furnace was placed within the boiler, as in the common marine boiler; and they proposed occasionally to excite the fire by bellows, worked by the piston, or crank of the engine, or by any plan found convenient. The cylinder of the engine was likewise immersed in the boiler. To guard against explosion a second safety valve was provided, not under the control of the engineer - a a plan which subsequently has been universally adopted in locomotive engine boilers, and which arrangement should be applied to steam boilers of every description.

Fig. 46. is a side and end elevation; a is the boiler, with flat ends, the fire being contained within the tube b, entirely surrounded with water; d the steam cylinder, placed nearly to the bottom of the boiler the waste steam passes into chimney c by eduction pipe e: the piston rod is attached to cross-head ff, which slides in vertical guides, from the ends of which the connecting rods 99 descend to the cranks on the axles of the fore wheels, and makes them revolve: h is the safety valve of the boiler.

In the year 1804 the inventors constructed an engine for moving railway carriages. This was the first steam engine applied to locomotive purposes in Britain. It was used successfully on the railroad at Merthyr Tydvil, in South Wales. The cylinder was placed horizontally, as in locomotives now used. The heads of the piston rod and connecting rod were divided, or forked, leaving room for the motion of the extremity of the crank, and giving motion to it fixed on an axle-tree on this axle cog-wheels were placed, working into cog-wheels on the axle of the hind wheels. This locomotive engine, which in many of the leading features was essentially the same as those now in use, had only one cylinder