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certainly. It will convey to the Committee more correctly my meaning, if I state that the quality of the road ought to be such, that, as the wheels roll over it, it should not suffer any change of its figure.
"This quality is strictly in conformity with the laws of science as relating to moving bodies ?— Strictly.
"According as I understand you, there is no difference of opinion among scientific men as to the necessity of having a road as hard as possible? —Yes, in order to offer the least possible resistance to the tractive force.
"What, in your opinion, is the proper way of getting rid of the elasticity in roads when constructing them ?—That is, I think, a question which involves a great deal of practical difficulty. It is quite clear that, casteris paribus, the thicker the crust of the road is the harder it will be, because a thick crust will not yield as much as a thin one.
"In laying out a line of road you would avoid, if possible, going over marshy ground ?—Clearly so; or at least, if I did go over it, I would take care to press it down, so as to destroy its marshy character, as we have done in the Chatmoss, on the Liverpool and Manchester railroad."
The explanation of the laws of motion, which has been given in this chapter, as applicable to the subject of road-making, and of the effect of an elastic substratum of a road in consuming the moving force, and adding to the horse's labour, is quite conclusive in showing how much at variance
with the first principles of science the following doctrines are, which are to be found in some modern publications.
"That a foundation or bottoming of large stones is unnecessary and injurious on any kind of subsoil."
"That the maximum strength or depth of metal requisite for any road is only ten inches."
"That the duration only, and not the condition of a road, depends upon the quality and nature of the material used."
"That free stone will make as good a road as any other kind of stone."
"That it is no matter whether the suhstratum be soft or hard." *
* The passages marked with inverted commas have been extracted from the publications of Mr. M'Adam.
As many persons advocate Mr. M'Adam's doctrine of elastic roads, it may serve to show the real value of it, by putting it in juxta-position with that of the celebrated natural philosopher, the late Professor Leslie.
Extract from the Evidence of Mr. MlAdam. Extract from Pro(Remarks Oh Road-making, p. 111.) fessor Leslie's
"Elements of Natural Philosophy."
"What depth of solid materials would "The resistance you think it right to put upon a road in which friction ocorder to repair it properly?—I should casions (to carthink that ten inches of well-consolidated riages) partakes of materials is equal to carry any thing. the nature of the
"That is, provided the substratum is resistance of fluids: sound ?—No: I should not care whether it consists of the the substratum was soft or hard: I should consumption of the rather prefer a soft one to a hard one. moving force or of
Mr. Wingrove, an eminent practical road surveyor, observes, in a Treatise on the Bath roads, after quoting the preceding passages from Mr. M'Adam's book, "that with respect to these opinions on road-making, nothing but the complete ignorance of the public upon all matters concerning road-making could ever have suffered rules, so contrary to every thing like sound principles, to have had a single moment of favourable considejation."*
The resistance produced by gravity, in checking the progress of a moving body on a road, is little or nothing when a road is horizontal, because as gravity acts in a direction perpendicular to the plane of the horizon, it neither accelerates nor retards the motion, t But when the road is not
"You don't mean to say you would the horse's labour, prefer a bog ?—If it was not such a bog occasioned by the as would not allow a man to walk over, I soft surface of the should prefer it road, and the con
"But must not the draught of a car- tinually depressing riage be much greater on a road which of t/te spongy and has a very soft foundation than on one elastic substrata of which is of a rocky foundation ?—I think the road." the difference would be very little indeed, because the yield of a good road on a soft foundation is not perceptible."
* Mr. Wingrove was for several years the surveyor of nearly all the roads in the neighbourhood of Bath. In 1825 the author accompanied him in making an inspection of them, and found the rules which Mr. Telford recommends had been most effectually acted upon throughout the whole of these roads, and that they had been brought to as high a state of improvement as the money which was allowed for them would admit of.
f Wood's Mechanics, p. 20.
horizontal, the power of gravity is a great impediment.
A mathematical illustration of the effect of gravity on hills is given in note D.
The resistance arising from the action of the air is very variable; in some cases, it acts powerfully; but as its influence is the same whether the road be bad or good, little need be here said on the subject: it will be sufficient to state, that by experiments detailed in Smeaton's Reports, it .was found that the force of the wind on a surface 1 foot square was 1 lb., when the velocity of the wind was 15 miles an hour, or what would be termed a brisk gale; 3 lbs. when the velocity was 25 miles an hour, or what would be termed a very brisk gale; 6 lbs. when the velocity was 35 miles per hour, or what might be termed a high wind; and 12 lbs. to the square foot when the velocity was 50 miles an hour, or what might be termed a storm. Supposing, therefore, that the surface of that part of a carriage acted upon by the direct influence of the wind to be 50 superficial feet, the resistance it will meet from a brisk gale of wind acting against it will be about 50 lbs. when the carriage is slowly moved; but if the carriage be supposed to move directly against the wind with a velocity of 10 miles an hour, and the wind to move with a velocity of 15 miles an hour, the resistance against the carriage will amount to 3 lbs. on the square foot, or 150 lbs. on the carriage, which is fully equal to the power which two horses should be required to exert, when moving with a velocity of 10 miles an hour. From this the difficulty is evident of driving stage coaches at a rapid rate against high winds.*
* The late M. Navier, one of the most scientific members of the French Institute, and Engineer in Chief of the Administration of the Ponts and Chaussdes, has borne testimony to the correctness of the principles and reasonings contained in this chapter, by having given a translation of the whole of it in his book on Roads. He says, "the large extract which I here insert contains that part of the Treatise of Sir Henry Parnell which has appeared to me to be the most interesting to French engineers, and which relates most immediately to the object in view." — Considerations sur les Principes de la Police du Roulage, et sur les Travaux dentretien des Routes. Par M. Navier. Paris, 1835.