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that the friction of wheels on the surface of a road will be reduced as much as possible.*
To comprehend thoroughly the great importance of making a regular and strong foundation, it should be borne in mind, that roads are structures that have to sustain great weights, and violent percussion; the same rules therefore ought to be followed with them as are followed with regard to other structures.
In building edifices which are to support great weights, whether a church, a house, or a bridge, the primary and indispensable consideration of the architect is to obtain a permanently firm and stable foundation. He well knows that unless this be first substantially made, no future dependence can be placed on the stability of the intended superstructure: but this most requisite precaution has but recently been attended to in the formation of roads, and only on those roads in Scotland, and between London and Holyhead, which have been under the direction of Mr. Telford.
If the foundation of a road be not sufficient and equal to the pressure it has to sustain, the whole fabric, though in other respects ever so well constructed, must fail in permanent stability, and its hardness will be imperfect on account of its elasticity.
Having now stated all that the rules of science relating to moving bodies suggest, in order to defend the principles of road-making, which have been laid down as those proper to be adopted, we shall proceed further to illustrate and support these principles, by reference to experiments of the force of traction on different kinds of roads. These experiments have been made with the machine invented by Mr. Macneill, which has been already mentioned, and which
* The mathematical illustration of the effect of friction on carriages is given in note C.
for their accuracy, in consequence of their having been carefully examined by several eminent civil engineers.
These experiments uniformly show, that the force of traction is, in every case, in an exact proportion to the strength and hardness of a road. The following are the results: on a well-made pavement, the power required to draw a waggon is 33 lbs.; on a road made with six inches of broken stone of great hardness, laid on a foundation of large stones, set in the form of a pavement, the power required is 46 lbs.; on a road made with a thick coating of broken stone, laid on earth, the power required is 65 lbs.; and on a road made with a thick coating of gravel, laid on earth, the
power required is 147 lbs. Thus it appears that the results of actual experiments fully correspond with those deduced from the laws of science.*
* The following is an extract from the evidence of Mr. Macneill before the Committee on Turnpike Roads (1836):
“Are you still satisfied of the principle of your machine being a correct one, as to ascertaining the draughts of carriages ? I am quite certain of its being perfectly correct, for we have tried it in some cases, and compared it with a weight hanging over a pully, and the results were the same. It is only in cer
It has been considered necessary to enter into these details in showing that no road can be cor
tain cases where a weight over a pulley could be applied ; it could not be done practically over a road of any length.
“ Has anything occurred as to the soundness of your principle being controverted by other engineers ?–No.
“ Is it generally adopted by them as a correct machine ? Yes, and referred to in very many cases.
In France there has been a petition to the Chamber of Deputies, founded on my experiments, relative to the mode of ascertaining the draught of carriages, and the saving by using springs.
“ You were the author of that algebraical calculation delivered in the Lords ?-Yes.
“ Does it correspond with the results made by the machine ? -It was founded on experiments made by the machine ; it was a formula that would give the power required to draw a carriage over a road in a section of that road, from data determined by experiments made by the machine.
“ Is your machine calculated to give the draught on setting the body in motion, or when it is in motion ?-Both.
“ Then it would appear that your former calculations, as to different effects of different roads on the draughts of carriages, are correct ?— Yes, quite correct; and they have been confirmed by very many experiments I have made since I was examined before the Committee of the House of Lords.
“ Then, in fact, the general conclusion is, that a road is good for its object, namely, of diminishing the draught of a carriage, in the proportion that it is hard and smooth ?-- The great advantages of the roads appearing by the machine is certainly in proportion to their solidity and their strength, and their want of yielding. If it could be a perfectly solid mass of stone or metal, the least resistance would be presented; that is shown both on stone tramways and on metal tramways and metal rails. There are some metal tramways laid in Glasgow on rather a steep hill, and it is not at all unusual for a horse to take from two to three tons; that arises merely from the saving in the resistance of the surface friction being lessened.
rectly called a good one unless it is so constructed as to be very strong and very hard, because all the main roads of the kingdom are still very defective in respect to strength and hardness. This is a fact which cannot be disputed; first, because there is always mixed up with the body of materials, which forms the crust of every road, a great quantity of earth ; secondly, because this crust is every where too thin ; and, thirdly, because it very frequently lies upon an elastic substratum. Although there may be exceptions, this may be taken as an accurate description.
Notwithstanding that all the roads are now much better than at any former period, and may deserve to be called good, in comparison with those of ten or fifteen years ago; when it is considered how much better they would be if they were reconstructed with a proper foundation coated with
“ That is, from the smoothness of the surface ?-Yes, from the smoothness and hardness.
“ So that if clean material of any road nine inches thick were properly beat down, that will not yield ?—Nine inches will yield
“What, on an old road of nine inches thick ?-Yes, with heavy waggons. One of the great advantages arising from Mr. Telford's system of forming roads by large stone pavements, is from the fact that one point is distributed, that the pressure of the wheels is distributed over a large space. The wheels of the carriage rest on, say two inches of surface, but that is carried to a large pitching stone below, which rests on the soil, and the weight is distributed over a large surface at the bottom; that is to say, over a surface a foot or nine inches long, and six or eight inches wide; it is lessened very much indeed on the surface that bears on the earth."
broken stones of great hardness, they should still be set down as being imperfect. Let
Let any trustee or surveyor who doubts this, reconstruct a mile of a road, now considered an excellent one, with a bottoming of pavement, coated with hard stones,
, and no stage coachman who shall drive over it will hesitate to bear testimony to the increased ease with which his horses do their work upon it.
The following extract from the examination of Dr. Lardner will serve to explain, and at the same time
support, the statements contained in the preceding pages.
“ On the elasticity of roads. “ Do you speak from experience on this point of elasticity ?-Not from experience, as an engineer, but only from having devoted a good deal of time to the consideration of this subject, and being perfectly acquainted with the experiments that have been made, and the experience we have had upon roads; and I also give that opinion (that a road should not be elastic) upon general scientific principles.
“ With reference to the general laws of motion ? -Undoubtedly; it is not a point about which any two scientific men can differ; there can be no difference of opinion about it. I mean that that quality is best for the surface of a road which will not permit it to alter its figure under the pressure of the wheels.
“ Then the degree of hardness will depend upon the degree in which elasticity is absent ?-Yes, ,