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The explanation which has been given in this chapter of the laws of motion, as applicable to the subject of road making, and particularly of the effect of an elastic substratum of a road, as stated by Professor Leslie, in consuming the moving force, and adding to the horse's labour, is quite conclusive in showing how much at variance to 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 substratum 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,
Mr. Wingrove, an eminent practical road surveyor, observes, in a Treatise on the Bath roads, after quoting these sentences, "that with
by putting it in juxta-position with that of the celebrated natural philosopher, the late Professor Leslie.
Extract from the Evidence of Mr. M'Adam. (Remarks on Road-Making, p. 111.)
"What depth of solid materials would you think it right to put upon a road in order to repair it properly?—I should think that ten inches of well consolidated materials is equal to carry any thing.
"That is, provided the substratum is sound?-No: I should not care whether the substratum was soft or hard: I should rather prefer a soft one to a hard
"You don't mean to say you would prefer a bog?—If it was not such a bog as would not allow a man to walk over, I should prefer it.
"But must not the draught of a carriage be much greater on a road which has a very soft foundation than on one which is of a rocky foundation?—I think the difference would be very little indeed, because the yield of a good road
Extract from Professor Leslie's "Elements of Natural Philosophy."
"The resistance which friction occasions (to carriages) partakes of the nature of the resistance of fluids: it consists of the consumption of the moving force, or of the horse's labour, occasioned by the soft surface of the road, and the continually depressing of the spongy and elastic substrata of the road."
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 consideration."*
3d, 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. † But when the road is not horizontal, the power of gravity is a great impediment.
A mathematical illustration of the effect of gravity on hills is given in note D.
4th, 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
* 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.
† Wood's Mechanics, p. 20.
whether the road be a bad or a good one, 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,