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pebbles, in such a way as to form a solid foundation for them, and support their surfaces at a uniform level in spite of their different thicknesses. Below all is a seconu bed of mortar filled with chip stone, and forming with it a solid mass, that at the present day is harder than the stones themselves, and probably attained this hardness soon after it was laid. The whole constitutes a body of upwards of three feet in thickness, and is almost every where elevated above the surface of the ground.
The roads of modern times are much less expensive in their construction ; they are also much less fitted for the purpose of easy conveyance either of passengers or of commodities. On the continent of Europe, particularly in France, the system of roads paved with flat stones in the middle, still continues, a relic probably of the Roman customs ; but so deficient in solidity are they, as to require constant repairs, and they are generally in such bad order that all light carriages prefer to travel upon the sides on the earth, except where this part of the road is rendered impracticable by the lodgement of water.
In the south east of England, the country furnishes, at small depths beneath the surface, beds of solid Aints. These appear to have been resorted to at a very early period, as materials for roads ; and when properly selected, they form perhaps the best and smoothest paths that exist in the world. To General Roy that country is indebted, in a great measure, for having been the first to discover that fragments of the rocks fou id in situ, in convenient positions and prepared for the purpose, were equally valuable as materials for roads with finty gravel. Yet the true principles of applying either of these substances were not understood ; and hence the very great variety in the quality of different roads, and even of the same road at different periods, arising from uncertain and vague views of the modes of making or repairing.
The true principles of road making have lately been laid down by M'Adam, upon a basis that cannot be controverted; and the introduction of these principles into general practice, is causing a rapid advance to perfection in the roads of England, already superior to those of every part of the continent of Europe, with the single exception of Sweden.
The superiority of the roads of England is owing in no small degree to their turnpike system, by which the cost of repairing the roads is thrown upon those who actually travel them, who in most cases contribute with pleasure. The same system has been partially introduced into the United States, but its most valuable feature has been neglected. The turnpikes of England are all public trusts, whereas in this country
they have been committed to private speculation. In both countries, the original capital has generally been furnished by the proprietors of the lands through which they pass; and for this the use of the road is itself frequently a sufficient compensation. If interest in addition is paid them, they may be considered to have made a very profitable investment; if there be any surplus, it goes to America to enhance these profits, while in England it is applied to increase the public accommodation. If the undertaking be unsuccessful among us, the road is abandoned, and frequently lost to the public; while in England in a similar event, the gates are still kept up, and the tolls applied, in addition to the statute labour, that is in such a case called in, to the repairs of the road.
Every road, however well constructed, is liable to a very considerable degree of friction. Upon a level road, this, however, is the only resistance; and in consequence, a horse that will not raise a weight of more than 187 pounds with a velocity of 2, miles per hour perpendicularly upwards, will draw fifteen hundred weight upon a well constructed carriage, itself weighing seven hundred weight, on a level road. But this power is rapidly diminished with a change in the inclination or in the smoothness of the road : while a man may mount perpendicularly up, as when upon a ladder, loaded with a considerable weight, a horse becomes powerless even to raise his own body upon a plane whose inclination exceeds 450. It is therefore a most important principle in laying out ordinary roads, that the change of level should be the least possible. Hence it is only in an absolutely level country, or one of a regular inclination in the required direction, that a road in a straight line ought to be admitted. Even in a gently undulating country, the road should deviate to the right or the left in such a manner, as to lie in one level line, or in as few planes of gentle inclination as possible. Is a ridge to be crossed, its lowest pass is to be sought, and the road to be conducted thither by approaches along the sides of the ridge, that if developed will form a plane of uniform inclination; and this inclination, if the road is to be travelled by loaded carriages, should never exceed one foot in forty, or an angle of 1° 30',
In applying these principles to a billy country, the distance measured horizontally will be frequently considerably increased, but this is a matter of no consequence whatsoever ; the effective length of a road for pleasure carriages, is measured by the time occupied in passing over it, and when used for the conveyance of heavy articles, it depends upon the compound ratio of the weight and the time. It will in some cases happen, that the distance actually travelled may be diminished, by deviating
from the straight line, for this distance is not the aerial horizontal line, but may be measured by the number of revolutions performed by the wheels of the carriage. More practical blunders have been committed in this country in this branch of civil engineering, than in any other. It appears to be adopted as a principle, that the direct aerial straight line is the only one that is admissible in theory, and that in practice, the deviation from this must be the least possible; when, in truth, the principle that should be received in preference is, that the road which will be travelled in the shortest time, between two places situated upon
the same level, is that which deviates least from a horizontal plane; and between two places of sufficient elevation, is that which approaches nearest to a plain of uniform inclination.
Other circumstances, besides those of difference of level and interposing ridges, must be taken into account; of these, the most important is, the nature of the soil, which will sometimes call for deviations of a still greater amount from the straight line. The convenience of the inhabitants, the position of old established settlements and villages, are also points that should never be neglected, whether we consider a road as a public enterprise, or a private speculation from which profit is anticipated.
The erroneous idea of a straight aerial line, is not only received as truth by country surveyors and road makers, but has crept into legislative enactments, and has, in many cases, been productive of essential evil. We have a most noted instance of this kind in a neighbouring state. A law granting a turnpike charter, required that it should be laid out in a straight line between its extreme points :-now, to draw a straight line upon the ground between two points not in sight of each other, is no easy problem in practical surveying, and, for a time, baffled the utmost skill of the persons engaged to lay out the route. This was, however, at last oyercome, and the road constructed on this line has for twelve years enjoyed the unenvied reputation of being the worst turnpike road in the United States, and one half of it, being remote from human habitation, is no longer travelled, a lateral route having been brought into competition with it to accommodate the inhabitants, and thus every chance of remuneration to the proprietors destroyed. This is probably an extreme case; but no person can travel in our country, without meeting, at every step, with instances of immense expenditure, applied unprofitably to the projectors, and disadvantageously to the public, from an ignorance of a few simple and fundamental principles. Nor is this ignorance confined
to those who have been deficient in the means of acquiring knowledge; it is to be found in national works, where scientific engineers have been engaged. The approach to our national military academy from the Hudson, constructed at vast expense, and under the direction of the corps of United States engineers, is hardly accessible for a loaded carriage; but if the learned professor of engineering shall annually, in his lectures, point it out to his pupils as a model of errors that are to be avoided, we cannot say but that the money has been well laid out, and the example most instructive.
The friction being the principal resistance that is to be overcome upon a well laid out road, it will be evident, that every diminution that can be effected in this cause of retardation, is important; hence the advantages of well finished and compact carriages, of smooth and hard roads; but the properties of smoothness and hardness may be carried to too great an extent, by causing injury to the horse, or depriving him of a firm footing. For this reason, the path of the horse, and of the wheels, may, when roads approximate to perfection, be made of different materials. The first approximation to a construction of this kind, is to be found in the pavement of the streets of Pisa, in which tracks for the wheels of carriages are formed of parallel rows of smooth hard stones, closely jointed, and firmly imbedded.* The railway introduced nearly a century since in the vicinity of the coal mines of Northumberland, is a more important and valuable application of this principle. In this original form of the railway, parallel rails of wood were laid from the mouth of the mine to the place of embarkation of the coal, which were faced with bars of wrought iron to receive the wheels of the wagons employed in the trade. Railways have been since much improved by the substitution of entire tracks of cast iron firmly placed upon stone bases, and innumerable instances of these are to be found in various parts of Great Britain. The advantage possessed by a railway over a well constructed road, when animal power is used as the propelling force, is great. We have already stated, that a horse, upon a good level road, draws a load of fifteen hundred weight; upon a railway, he will draw, with equal ease, and at the same velocity, a load of ten tons, giving the railway the advantage of seven to one. But as roads are very rarely free from ascents and descents, the effective proportion between a
* This example is well worthy of imitation in the pavements of great cities, and, it is to be hoped, will not be neglected in the contemplated renewal of the pavement of Broadway.
railway and the best ordinary roads, may be fairly considered as equivalent in practice to a ratio of ten to one. Wherever, then, the traffic upon a road is sufficient to pay the interest of the cost of construction, a railway may be substituted for it, and even where animal power is used, to very great advantage.
But when animal power is the sole disposable agent, there are other modes of conveyance far more advantageous than either roads or railways. The load that would require an enormous force to lift it perpendicularly upwards, and a very great one to drag it along a horizontal plane, is rendered comparatively easy to remove, if placed upon a well constructed Carriage; and this advantage is, as we have seen, increased in a decimal ratio if a railway be substituted. But if, instead of a carriage, it be placed upon a vessel floating in a liquid, its whole weight is supported by the hydrostatic pressure of the liquid, both this and the friction are at once annihilated, and we have in their lieu to overcome the resistance of a fluid. Such a resistance is, at small velocities, far less than the friction, and may, therefore, be much more readily overcome by the application of animal strength. There are even many cases where a much cheaper agent may be substituted in the propulsion of vessels. In the infancy of navigation, paddles first, and then oars, were employed. The resistance opposed by the wind, to a course contrary to its direction, and the aid that it afforded when favourable, led, at a period more early than the date of our oldest profane histories, to the introduction of the sail. But the use of this was, until very recently, limited, and its most important advantages unknown; spread nearly at right angles to the plane of the keel, it was only employed in the case of favourable winds; when their direction was opposed to the desired course, it was furled, and the vessel either abandoned to the influence of the wind, or urged against it with painful efforts by means of oars. Such was the system of navigation employed by the ancient nations, and such were the only methods in use in Europe up to a time little prior to the date of the discove
of America, That the wind itself, acting upon a vessel of a peculiar form, floating in a fluid whose law of action presented an intense resistance to motion in any direction other than the plane of the keel, afforded the means of progress in its very teeth, was not understood, until mathematical and mechanical science, the precursors of the revival of letters and the fine arts, began to be studied in Italy. In this country, and at this period, the nautical manœuvre that is technically called beating, appears to have originated ; and from this discovery, connected with that of the mariner's compass, an invention of