Report from the Select Committee on Steam Carriages/Gilbert
Davies Gilbert, Esquire, a Member of the Committee; Examined.
Have you paid any attention to the general nature and advantages of wheels and springs for Carriages, the draft of cattle, and the form of roads?—I paid considerable attention to it during the sitting of a Committee of this House about Twenty years ago, of which Sir John Sinclair was Chairman, and I then drew up some Observations on the nature of wheels and springs on roads, which, with some alterations, I printed in the Eighteenth volume of the Journal of Sciences, and which I would beg to deliver in to the Committee as the result of my observations on the subject.
[The same was read as follows:]
"Taking wheels completely in the abstract, they must be considered as answering two different purposes.
"First, they transfer the friction which would take place between a sliding body and the comparatively rough uneven surface over which it slides, to the smooth oiled peripheries of the axis and box, where the absolute quantity of the friction as opposing resistance is also diminished by leverage, in the proportion of the wheel to that of the axis.
"Secondly, they procure mechanical advantage for overcoming obstacles in proportion to the square roots of their diameters, when the obstacles are relatively small, by increasing the time in that ratio, during which the wheel ascends; and they pass over small transverse ruts, hollows or pits, with an absolute advantage of not sinking, proportionate to their diameters, and with a mechanical one as before, proportionate to the square roots of their diameters, consequently, wheels thus considered, can not be too large; in practice, however, they are limited by weight, by expence, and by convenience. "With reference to the preservation of roads, wheels should be made wide, and so constructed as to allow of the whole breadth bearing at once; and every portion in contact with the ground should roll on it without the least dragging or slide; but, it is evident from the well-known properties of the cycloid, that the above conditions cannot unite unless the roads are perfectly hard, smooth and flat; and, unless the fellies of the wheels, with their tires, are accurately portions of a cylinder; these forms, therefore, of roads and of wheels are the models towards which they should always approximate.
"Roads were heretofore made with a transverse curvature to throw off water, and in that case it seems evident that the peripheries of the wheels should, in their transverse sections, become tangents to this curve, from whence arose the necessity for dishing wheels, and for bending the axes, which contrivances gave some incidental advantage for turning, for protecting the nave, and by affording room for increased stowage above. But recent experience having proved that the curved form of roads is wholly inadequate for obtaining the end proposed, since the smallest rut intercepts the lateral flow of the water; and, that the barrel shape confines Carriages to the middle of the way, and thereby occasions these very ruts; roads are now laid flat. Carriages drive indifferent over every part, the wear is uniform, and not even the appearance of a longitudinal furrow is to be seen. It may, therefore, confidently be hoped that wheels approaching to the cylindrical form will soon find their way into general use.
"The line of traction is mechanically best disposed when it lies exactly parallel to the direction of motion, and its power is diminished at any inclination of that line in the proportions of the cosine of the angle to radius. When obstacles frequently occur, it had better perhaps receive a small inclination upwards, for the purpose of acting with most advantage when those are to be overcome. But it is probable that different animals exert their strengths most advantageously in different directions, and therefore practice alone can determine what precise inclination of this line is best adapted to horses, and what to oxen. These considerations are, however, only applicable to cattle drawing immediately at the Carriage; and the convenience of this draft, as connected with the insertion of the lipe of traction which continued, ought to pass through the axis of the wheels, introduces another limit to their size.
"Springs were in all likelihood applied at first to Carriages, with no other view than to accommodate travellers. They have since been found to answer several important ends. They convert all percussion into mere increase of pressure; that is, the collision of two hard bodies is changed by the interposition of one that is elastic, into a mere accession of weight. Thus the Carriage is preserved from injury, and the materials of the road are not broken: and in surmounting obstacles, instead of the whole Carriage with its load being lifted over, the springs allow the wheels to rise, while the weights suspended upon them are scarcely moved from their horizontal level. So that, if the whole of the weight could be supported on the springs, and all the other parts supposed to be devoid of inertia, while the springs themselves were very long, and extremely flexible, this consequence would clearly follow, however much it may wear the appearance of a paradox, that such a Carriage may be drawn over a road abounding in small:obstacles without agitation, and without any material addition being made to the moving power.or draft. It seems therefore probable, that under certain modifications of form and material, springs may be applied with advantage to the very heaviest waggons, and consequently, if any, fiscal regulations exist, either in regard to the public revenue or to local taxation, tending to discourage the use of springs, they should forthwith be removed.
"Although the smoothness of roads, and the application of springs are beneficial to all Carriages, and to all rates of travelling, yet they are eminently so in cases of swift conveyance, since obstacles when springs are not interposed, require an additional force to surmount them beyond the regular draft, equal to the weight of the load multiplied by the sine of the angle intercepted on the periphery of the wheel between the points in contact with the ground and with the obstacle, and therefore proportionate to the square of its height; and a still further force, many times greater than the former when the velocity is considerable, to overcome the inertia, and this increases with the height of the obstacle, and with the rapidity of the motion both squared. But, when springs are used, this latter part, by far the most important, almost entirely disappears, and their beneficial effects, in obviating the injuries of percussion, are proportionate also to the velocities squared.
"The advantages consequent to the draft, from suspending heavy baggage on the springs, were first generally perceived about forty years since on the introduction of mail coaches; then baskets and boots were removed, and their contents were beaped on the top of the Carriage. The accidental circumstance, however, of the weight being thus placed at a considerable elevation, gave occasion to a prejudice, the cause of innumerable accidents, and which has not, up to the present time, entirely lost its influence; yet a moment's consideration must be sufficient to convince any one, that when the body of a Carriage is attached to certain given points, no other effect can possibly be produced by raising or by depressing the weight within it, than to create a greater or less tendency to overturn."
The extensive use of waggons suspended on springs, for conveying heavy articles, introduced within these two or three last years, will form an epoch in the history of internal land communication not much inferior perhaps in importance to that when mail coaches were first adopted; and the extension of vans, in so short a time, to places the most remote from the Metropolis, induces a hope and expectation that as roads improve, the means of preserving them will improve also, possibly in an equal degree; so that permanence and consequent cheapness, in addition to facility of conveyance, will be distinguished features of the McAdam system.
I have made some further remarks, which I would beg to deliver in also, tending to point out particularly the advantage of steam conveyance when the rate of travelling is great: I would beg to add, that it appears to me extremely difficult to lay down any general rule which would be applicable to all situations and all roads, inasmuch as they vary with the nature of the materials: that up to a certain weight, proportionate to the corresponding width of the wheel, it is probable that the injury to any road may be very little, but that beyond a certain weight, compared again with a corresponding breadth of the wheels, the materials would be entirely crushed and the road totally destroyed; therefore it follows, that even on all roads there must be a limit to the weight of Carriages, as it is quite impossible that a wheel of enormous breadth could bear uniformly on all its surface. For instance, where trains of artillery are drawn over roads, the excess of their weight beyond what materials are capable of sustaining, has been found sufficient for grinding them to powder. "The slow conveyance of heavy weights may perhaps be effected by steam on well-made and nearly level roads, so as to supersede the use of horses; but steam power is eminently useful for producing great velocities. It was last year determined by the Society of Civil Engineers, after much inquiry and discussion, that the expence of conveying Carriages drawn by horses was at its minimum when the rate of travelling equalled about three miles an hour, and that expence increased up to the practical limit of speed, nearly as the velocity; including the greater price of horses adapted to swift driving, their increased feed and attendance, the reduced length of their stages, and, with every precaution, the short period of their services; on the contrary, friction being a given quantity as well as the force requisite for impelling a given weight up a given ascent, the power required for moving Steam Carriages on a Rail-way remains theoretically independent of its speed, and practically increases but a very little, in consequence of resistances from the atmosphere, slight impacts against the wheels, inertia of the reciprocating piston, &c. The expenditure of what I have termed Efficiency, is as actual force multiplied by the velocity, and the consumption of fuel in a given time will be in the same proportion, but the time of performing a given distance being inversely as the velocity, the expenditure of fuel will theoretically be constant for a given distance, and very nearly so in practice. The power requisite for moving bodies through water is in the opposite extreme; here, the mechanical resistance of the fluid increases with the square of the velocity, as do the elevation of the water at the prow and its depression at the stern. The cars or paddles must therefore preserve a constant ratio to the velocity of the vessel; and the force applied will consequently vary as the squares of the velocity, and the expenditure of Efficiency being as the force multiplied by the velocity; the consumption of fuel will be as the cube of the velocity in a given time, or as the square of the velocity on a given space; and I have ascertained from the records of voyages performed by Steam vessels, that the law is nearly correct in practice: hence the great power required for such Steam vessels as are constructed not merely for speed, but also to set at defiance the opposition of winds and seas; while, on the contrary, a very small power will be found sufficient for moving ships of the largest dimensions through the water, at the rate of two or three miles an hour, when their sails are rendered useless by continued calms."