18. It is evident that the round form of fire-box possesses great advantages over the square fire-box: first, it is much safer than the square fire-box, being made nearly in that shape which an excess of pressure beyond its stiffness would tend to bring it to, if made in any other form; moreover, the safety of the boiler with a square fire-box is nearly dependent on the strength, individually, of each of the stays which is fixed in it, (of which there are a great number,) whereas the pressure in the round fire-box is borne equally by the whole area of the plates of which it is composed: again, the corners in the square fire-box, in which the combustion is always languid, and consequently injurious, are avoided in the round fire-box.
19. A lead plug is placed at the culminant point of the round fire-box, and will therefore melt before any other part is left dry, and, as the top row) tubes is placed two or three inches below the culminant point, it is almost certain that the extinction of the fire will prevent the tubes being burnt; but, in a fire-box with a flat top, the melting of the lead would only occur when the whole surface was dry, and probably injured.
20. It is admitted that a locomotive engine should be as light as is consistent with great strength, simple in its construction, composed of as few parts as possible, and that the greatest regard should be had to the diminution of friction; it is thence obvious that four wheels must be preferable to six, provided they carry the engine with the same steadiness.
"The proviso at the end of this paragraph is a very important one, but it is not secured in the four-wheeled engine; for if that has less friction than the six-wheeled engines, why do the engines on the London and Birmingham railway consume more coke per carriage per mile than those on the Liverpool and Manchester railway ? Eighteen months or two years ago, the difference was rather more than two to one, and is still very considerable."
21. The use of six wheels originated, (as we have before shown,) in the necessity of supporting the large and heavy fire-box, which was not sufficiently balanced by the smoke-box end; but no such necessity can exist in the locomotives made .according to the accompanying plan, as the weight is nearly equally distributed on the front and hind wheels, and not only would two additional wheels be useless, but they would be prejudicial and dangerous when the engines are travelling upon curves.
"This paragraph (21) commences with a misstatement, which has been already noticed. The London and Brighton Railway Company can speak to the latter part, as a man from Messrs. Bury & Co.'s has been two or three months putting a third pair of wheels (which were sent from Liverpool,) to each of the five or six engines made by the firm for that line; one of which engines, previous to the additional pair of wheels being put under it, caused a great loss of life not long ago."
22. A four-wheeled engine travelling upon a curve is driven, by the direct application of the moving power, towards the outside of the curve; but, as the wheels are rather conical, the large diameter of the cone will ride on the outside rail, while the smaller diameter of the opposite wheel will bear on the inside rail, and this difference, (as the outside rail is longer.than the inside one,) will allow the wheels to revolve without slipping or grinding.
"Another notable discovery in mechanics! Hitherto it has been always understood, that for the conical form of the wheels to produce the effect here described the axles must be radii."
23. With an engine upon six wheels, if the two leading wheels assumed this position, the others would necessarily be dragged after them; but a still more important point is, that the angle which the centre line of the locomotive forms with the tangent of the curve in which it is caused to move, is much greater with six wheels than with four, so that the flange of the wheel presses more against the rail with the former than with the latter engine.
24. The pressure against the outside rail, arising from this cause, will be in direct proportion to the distance between the front and hind axle of either engine, so that it will be as 10 to 6.
"'Mechanicus' in the Railway Times, has shown the reverse to be the case. That the lateral motion of the four-wheeled engine will bend the rails, was shown by the accidents on the Eastern Counties, and the Paris and Versailles Left Bank railways, where the rails were bent in a straight part of the line. I believe no instance has been known of a six-wheeled engine bending rails."
25. This pressure and consequent friction is still further increased by the action of the middle wheel, which tends to ride on the same curve as the front and hind wheels, but is prevented from doing so by being in a straight line between the two, and is thus forced to move laterally between the chord and the circumference of the curve.
26. The friction arising from this lateral motion further presses the engine against the outside rail. Thus the four-wheeled locomotive has, in proportion, a greater weight on the front wheels, it presses less against the outside rail, and offers much less friction when travelling on curves; hence, it has less tendency to be thrown off the rails, it is more simple in its construction, less expensive in repairs, on account of this simplicity, and the smaller cost of it fully justifies the directors of the several railways who have given the preference to this description of engine.
"It is notoriously untrue that the four-wheeled engine has less tendency to be thrown off the rails than the six-wheeled engine, as the Liverpool and Manchester, the London and Brighton, the Paris and Versailles Left Bank, and especially the Eastern Counties Railway Companies can well attest. Messrs. Bury and Co.'s engines are by no means as simple as the modern six-wheeled engines, and cannot be kept in order at so small a cost. With reference to the original cost, if the statement of an Eastern Counties' director, in a letter published in the Railway Times about eighteen months back, may be depended upon, Messrs. Bury and Co.'s charge for a four-wheeled engine with an iron fire-box (for I understand they will make no other) was 501. more than Messrs. Sharpe, Roberts, & Co. then charged for a six-wheeled engine of the same size with a copper fire-box. The North Midland Railway Company can give some information concerning iron fire-boxes, as they have had to put copper ones in lieu of the iron ones made by Messrs. Bury & Co."