I will now, in the order in which I have set them down, give you my opinion on each of these points of construction.

1. As the general course of the road, excepting where it crosses a valley, is carried along the side of the mountains, the general section will appear as in the plate. To the width of the road there can be no objection (A),* nor

2. To the width of the pavement. But to the construction in every other point, I shall take the liberty of offering such objections as both theory and experience have suggested, and as in a great many instances have been found solid in my own professional practice.

The idea of covering a road with stones of different sizes has, I believe, arisen originally in the expectation, that the soft clay or, earth, which in most instances constitutes the natural soil, would be less easily penetrated by large, (especially by flat stones) laid immediately upon it, than by smaller stones, such as necessarily form the surface of paved roads, for the sake of procuring a tolerably regular plane upon which to travel. This idea has afterwards been strengthened by the economy of the practice; the quantity of labour in breaking the stone being thereby diminished. But the expectations both of utility and economy are perfectly fallacious. In the first place, nothing is so certain, as that the large stones, let them be ever so carefully laid out at the bottom, will, in time, conic up to the top, and that the small stones will all go to the bottom: and this effect will take place, sooner or later, exactly in the proportion in which the road is much, or little, used; and also in the proportion in which the size of the large stones exceeds that of the small ones. At first sight this may appear extraordinary, but the effect is natural and unavoidable. It occurs thus: A heavy load presses upon the new road, and of course moves all the stones down to the lowest (B), the small stones at the top descend into the interstices, which are thus opened. Every new pressure and motion continues this process. If the lower stratum consists of stones which are large and flat, they will get to the top sooner than round stones. A very familiar example will elucidate this process. Every housekeeper, in breaking a loaf of sugar, shakes the broken pieces in a box. Those that are too large, soon come to the top and are broken smaller. In shaking chestnuts in a basket, the largest will soon be at the top, and exactly on the same principle. If it were not invidious, I would point out several turnpike roads, or parts of turnpike roads, in Maryland and Pennsylvania, which prove the truth of this fact. As to economy, (C) the calculation is in the next place quite as bad a one. Such a road soon wants repair, and no repair, excepting that of breaking up the large stones as fast as they appear, is an adequate one. These large stones necessarily range themselves on each side of the ruts, until they work themselves loose, and then they lie in the horse path. Such a turnpike is soon fit only for slow heavy waggons, and for those it is a very bad one.

Now in the United States' road, the lower stratum of stones will consist, at an average, of pieces weighing each more than 12 times as much as each of the pieces that constitute the upper stratum : or in other words, that have more than 12 times the volume, or bulk. Of course, many years will not elapse, before this road will be exceedingly out of repair and covered with loose stones of an average diameter of seven inches. In a level country this would be a less evil than among the mountains. Every obstruction of seven inches is a very serious additional difficulty in an ascent of four and a half degrees, to a loaded waggon the greatest radius of whose wheels is probably only 2 feet 9 inches.

3. In all roads whatever, it is an error to raise the centre above the shoulders of the road. If there are instances in which it is useful, I have not yet seen them. In a road merely thrown upout of the common soil, where the ground is very level lengthwise of the road, it is perhaps as well to raise the centre, because it will soon be beaten flat in dry weather by being chiefly used, and in wet weather, when the ruts or tracks are deep it is of no consequence, because the water cannot run off rapidly : but in all other cases, the convexity of the road is assuredly injurious.

In respect to their level, lengthwise, there can be only two kinds of roads. The first, such as are perfectly level, of which the instances are in the United States chiefly confined to sandy situations ; the second, such as arc inclined more or less to the horizon, and which in turnpike roads are permitted by law seldom to exceed in acclivity, an angle of four and a half degrees.

In the first kind of roads, that is in level roads if in any, it might be supposed that a convex surface would be useful, because the water falling upon the road, might be thereby carried into the ditches on each side of it. And this would happen without injury, if the surface were perfectly hard, and no ruts or tracks were cut into it by carriage wheels. But as the construction of the road, and the law regulating the passing of meeting carriages occasions the sides of a convex road to be used, more than the centre, the outer wheels will always bear the principal part of the load and of course cut the deepest track. If the road be perfectly horizontal, lengthwise, the water which falls on the road will run across the road into the outer ruts on each side, which are the lowest and the .deepest, and stand in them until it finds vent on to the shoulder in a place accidentally lower. The whole water of the road on each side for some distance, being discharged through a few of these openings, the road will be gullied across its edges— as daily observations may shew. But if the road were perfectly flat, there would not be cut so easily a leading rut on the edge, and the water would run off as fast as it rose above the surface in innumerable places, and much less rapidly. It would therefore gully much less, and nothing is so certain, as that, of roads entirely unimproved, the flattest parts arc certainly not the worst, because they hare the fewest gullies across them, unless the ground is naturally boggy.

It would be out of place here to give instances for the construction of roads in countries which are not sandy and yet perfectly level. I will only remark, that to raise the whole artificial part of the road a foot above the common level, to lay it flat across, and to effect a very small alternate rise and fall at short distances longitudinally would be a mode perfectly efficient—as very ample experience has proved. (D)

In roads however that have a natural inclination to the horizon, it is difficult to conceive, how the idea of convexity ever obtained admittance into practice. The makers of roads appear to have been more anxious to get rid of rainwater than on any other account. It is not pleasant to drive or ride through ponds or puddles in the public highway: but there need not be any fear, that they will abound or exist, if the surface of the road be well and equally laid, even- if the road were quite flat, provided it were raised above the level of the land on each side. But in a road which has a longitudinal declivity the only danger is, that the water will run off too fast, and carry with it all the small particles of hard stone, which filling up the interstices of the larger pieces, fix them firmly in their places, and which convexing them in some degree diminish the friction of the wheels and preserve the road. There cannot possibly be any occasion to turn off these particles from the road altogether by a lateral (producing with the longitudinal declivity an oblique) current of water, and of course an oblique gully, even if the road could possibly be quite smooth and free from tracks and ruts. But as no road, and least of all, a convex road can be free from ruts, while narrow wheeled waggons are used, the convexity of the road becomes useless at least. For the water will run from the centre into the first rut that it finds and continue to follow it until it finds some vent sideways, accidental or artificial. Since then the convexity of the road cannot perform the service expected from it, on account of the longitudinal tracks, the actual evils it creates ought surely to be avoided. The principal of these is the unequal bearing of the load of carriages upon their wheels, thereby wearing the lower side of the road, and forcing—by the cutting of leading ruts;—all the waggons to follow the same track, in which track there will be always chuck holes* in the lower side, whenever a large stone happens to be.

(* In the middle states, chuck holes, are sudden and deep depressions in the rut, and are always found opposite to the root of a tree, an old stump, on firmly fixed on the upper. It would be certainly a more rational construction to build up the shoulders of the road firmly with large stones rather higher than the road itself, and then to fill in perfectly level across with equal small stones. A road so constructed would retain its materials much better and longer than the other, especially if care were taken, to prevent as much as possible a very rapid discharge of the rain water from it.)

4. Contiguous to the pavement there is to be a ditch. This ditch must be certainly intended to catch the water coming down from the sides of the mountain, on the slope of which the road is made. To get into the ditch then, the water must pass over 4 or 5 feet of unpaved soil, the pavement being directed to be made in the centre of the 30 feet, which are to be levelled. I have understood, that the road continues in many parts upon one uninterrupted ascent of a mile and upwards. If this inclination should be of four and a half degrees and regular, it may easily be imagined, what will be the effect of a current of water running over the soft soil for a mile, and not only increasing as it descends in velocity, but also in volume. The bottom of the ditch would soon assume the parabolic line which the slope of all mountains acquire from the wash of ram water ; and the pavement would gradually be undermined and tumble into it. But I presume that there will be as usual, stops thrown up to guide the current across the road at proper distances, which, although not provided for in the contracts, is so far a practical remedy for the evil. But it must, even if this be done, be apprehended, that in time a gully will be formed on the upper side of the road; and this gully will take place on the soil between the ditch and the hill, making the whole of the upper unpaved level one irregular ditch.

Having thus objected to the whole construction of the road, you have a right to expect, that I should point out, what, in my opinion, it ought to be. This I will not hesitate to do, with the most perfect candor and freedom.

1. The road being cleared 30 feet in width, the upper boundary-of the road should be so laid, as that the earth to be cut from the slope of the hill will be sufficient to raise the lower or forced side higher than the other; the surface inclining towards the hill about 1 foot six inches. The made ground will settle so as to become perhaps nearly level, but it should always be rather higher than the opposite side to prevent the water from running across the road. The cut or. the hill side should be, in general, perpendicular, if the nature of the soil is tolerably tenacious. A perpendicular bank will stand much better, than any slope over which the rain water produces a current.

2. A drain ought then to be cut above the road upon the hillside, about 12. feet from the edge of the bank, and about a fort deep, and the earth thrown towards the road, in order to catch wash of the hill, and to prevent its ever reaching the road judgment is required in cutting these drains. Whenever opportunity offers, the drain should be turned down the lull from road, so as to throw off the water among the trees and a new be commenced. (See plate.) In many cases, this is impracticable ; but wherever the ravines in the slope of the hill, it may be done to advantage..wash of the slope will, however, at last require to be discharged across the road into the valley. Which should always be a small drain or tunnel, instead of an oblique draft, or sink at the road—than which nothing can be a greater nuisance, or a contemptible means of economy, if indeed any thing is save the contrivance. The use of resting ascending teams is plea —valeat quantum valere potest.

This upper drain on the slope of the hill is of the importance, and cannot, without injury, be omitted. It is an of very small expense, and being confined to no particular form, may easily be repaired or shifted, should the rei break over it. No very accurate rule can be laid down of the manner of t ducting it, as the numerous varieties of situation which must suggest as many, both in the length and direction of drains.

3. The pavement of the road of 20 feet in width, should the be laid entirely on the hill side of its width, measuring from the perpendicular bank, so that there will remain ten feet of unpaved road on the other side. This will afford an excellent summer road; whereas, if the road be paved in the middle, and only five feet remain unpaved, no carriage whatever can travel upon it.

If the pavement be thus laid close up to the bank, no gully be made between the road and the hill—for the water that falls between the guard ditch upon the slope, and the road, is of very little importance, and being discharged over the hard pavement, the road no injury.

There is no doubt, but that in the course of years, earth from the perpendicular slopes will fall on to the road; but nothing is then more easily removed, and by degrees the bank will gain a slope at which it will stand, and be covered with vegetation. If the soil be naturally good, this will happen in three or four years. 4. The pavement should be laid perfectly level across the road, or rather, if at all, inclining a little towards the hill. All the stones should be broken to the same size. Stones that will pass through a 3 inch ring are of a very good size. But it were better that they should be even larger, than that larger stones should be laid first,-and smaller stones upon them. That they should be all equal, is much more essential, than that they should either be very small or the stratum be very thick. A road of equal stones laid 8 inches thick, will last longer without requiring repair, than a road of two strata of different sizes of a foot thick. And a little reflection will convince any one, that it must be so. A shoulder of large stones would indeed be very useful; for it is important to prevent the road from spreading ; but a shoulder of earth is probably sufficient, and admits of an easier passage from the paved to the unpavcd road.

5. Culverts, or if it must be so, stops (these stops are admirable contrivances to make a ditch of the summer road) across the road, must be made to take off the rain water which falls upon the road, and also the wash of the hills below the guard drains. These must be placed where the judgment of the superintendent of the road may direct, according to circumstances. But, as I said before, drains or culverts should generally be used, and always to discharge the water of the guard drains—and where flat stones abound, their expense is very trifling.

6. The principal part of the U. States' road being carried upon the slopes of mountains, the section of the road I propose would be thus. (See plate.) But where it is to be carried on an embankment over a valley, over ground which docs not rise on one side above, and on the other fall below it, or over a ridge, then the pavement should be in. the centre of the road, and the whole road be 40 feet wide at least. But in the case of a perfect level across the road, there must be a ditch on both sides at the extreme of the 30 feet, for the purpose of preventing water from running on to the road, and to carry off the drainings of the road itself. The paved part of the road may be laid six or eight inches higher than the sides, if the road be level longitudinally: otherwise it is not necessary. I have now performed my promise, although in a manner much less full and systematical than I could have wished. I have only to add a request, that you will excuse the marks of haste which you may observe, and receive this letter as a proof of my anxiety to be useful to the public.

I am, very respectfully, yours,

B. H. LATROBE, Surveyor of the public buildings of the U. States.

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