square inch of wire cable

Tension resulting from weight of water upon 1 square inch of anchor-chains

.

Pressure resulting from water upon a pyramid

2090 tons.

14800 lbs.

11000 "

137 tons.

Do.

upon one superficial foot

18400 lbs."

ROADS.

634. IN establishing a line of internal communication of any character, whether it be an ordinary road, railroad, or canal, the main considerations to which the attention of the engineer must be directed in the outset are-1, the probable character and amount of traffic over the line; 2, the wants of the community in the neighborhood of the line; 3, the natural features of the country, between the points of arrival and departure, as regards their adaptation to the proposed communication.

As the last point alone comes exclusively within the province of the engineer's art, and within the limits prescribed to this work, attention will be confined solely to its consideration.

635. Reconnaissance. A thorough examination and study of the ground by the eye, termed a reconnaissance, is an indispensable preliminary to any more accurate and minute survey by instruments, to avoid loss of time, as by this more rapid operation any ground unsuitable for the proposed line will be as certainly detected by a person of some experience, as it could be by the slow process of an instrumental survey. Before however proceeding to make a reconnaissance, a careful inspection of the general maps of that portion of the country through which the communication is to pass, will facilitate, and may considerably abridge, the labors of the engineer; as from the natural features laid down upon them, particularly the direction of the watercourses, he will at once be able to detect those points which will be favorable, or otherwise, to the general direction selected for the line. This will be sufficiently evident when it is considered -1, that the water-courses are necessarily the lowest lines of the valleys through which they flow, and that their direction must also be that of the lines of greatest declivity of their respective valleys; 2, that from the position of the water-courses the position also of the high grounds by which they are separated naturally follows, as well as the approximate position at least of the ridges, or highest lines of the high grounds, which separate their opposite slopes, and which are at the same time the lines of greatest declivity common to these slopes, as the water-courses are the corresponding lines of the slopes that form the valleys.

Keeping these facts (which are susceptible of rigid mathematical demonstration) in view, it will be practicable, from a careful examination of an ordinary general map, if accurately constructed, not only to trace, with considerable accuracy, the general direc

tion of the ridges from having that of the water-courses, but also to detect those depressions in them which will be favorable to the passage of a communication intended to connect two main or two secondary valleys. The following illustrations may serve to place this subject in a clearer aspect.

the

If, for example, it be found that on any portion of a map water-courses seem to diverge from or converge towards one point, it will be evident that the ground in the first case must be the common source or supply of the water-courses, and therefore the highest; and in the second case that it is the lowest, and forms their common recipient.

If two water-courses flow in opposite directions from a common point, it will show that this is the point from which they derive their common supply, at the head of their respective valleys, and that it must be fed by the slopes of high grounds above this point; or, in other words, that the valleys of the two water-courses are separated by a chain of high grounds, which, at the point where it crosses them, presents a depression in its ridge, which would be the natural position for a communication connecting the two valleys.

If two water-courses flow in the same direction and parallel to each other, it will simply indicate a general inclination of the ridge between them, in the same direction as that of the watercourses. The ridge, however, may present in its course elevations and depressions, which will be indicated by the points in which the water-courses of the secondary valleys, on each side of it, intersect each other on it; and these will be the lowest points at which lines of communication, through the secondary valleys, connecting the main water-courses, would cross the dividing ridge.

If two water-courses flow in the same direction, and parallel to each other, and then at a certain point assume divergent directions, it will indicate that this is the lowest point of the ridge betweeen them.

If two water-courses flow in parallel but opposite directions, depressions in the ridge between them will be shown by the meeting of the water-courses of the secondary valleys on the ridge; or by an approach towards each other, at any point, of the two principal water-courses.

Furnished with the data obtained from the maps, the character of the ground should be carefully studied both ways by the engineer, first from the point of departure to that of arrival, and then returning from the latter to the former, as without this double traverse natural features of essential importance might escape the eye.

636. Surveys. From the results of the reconnaissance, the

engineer will be able to direct understandingly the requisite surveys, which consist in measuring the lengths, determining the directions, and ascertaining both the longitudinal and cross levels of the different routes, or, as they are termed, trial lines, with sufficient accuracy to enable him to make a comparative estimate both of their practicability and cost. As the expense of making the requisite surveys is usually but a small item compared with that of constructing the communication, no labor should be spared in running every practicable line, as otherwise natural features might be overlooked which might have an important influence on the cost of construction.

637. Map and Memoir. The results of the surveys are accurately embodied in a map exhibiting minutely the topographical features and sections of the different trial lines, and in a memoir which should contain a particular description of those features of the ground that cannot be shown on a map, with all such information on other points that may be regarded as favorable, or otherwise, to the proposed communication; as, for example, the nature of the soil, that of the water-courses met with, &c., &c.

638. Location of common Roads. In selecting among the different trial-lines of the survey the one most suitable to a common road, the engineer is less restricted, from the nature of the conveyance used, than in any other kind of communication. The main points to which his attention should be confined are―1, to connect the points of arrival and departure by the most direct, or shortest line; 2, to avoid unnecessary ascents and descents, or, in other words, to reduce the ascents and descents to the smallest practicable limit; 3, to adopt such suitable slopes, or gradients, for the axis, or centre line of the road, as the nature of the conveyance may demand; 4, to give the axis such a position, with regard to the surface of the ground and the natural obstacles to be overcome, that the cost of construction for the excavations and embankments required by the gradients, and for the bridges and other accessories, shall be reduced to the lowest amount.

639. Deviations from the right line drawn on the map, between the points of arrival and departure, will be often demanded by the natural features of the ground. In passing the dividing ridges of main, or secondary valleys, for example, it will frequently be found more advantageous, both for the most suitable gradients, and to diminish the amount of excavation and embankment, to cross the ridge at a lower point than the one in which it is intersected by the right line, deviating from the right line either towards the head, or upper part of the valley, or towards its outlet, according to the advantages presented by the natural features of the ground, both for reducing the gradients and the amount of excavation and embankment.

Where the right line intersects either a marsh, or water-course, it may be found less expensive to change the direction, avoiding the marsh, or intersecting the water-course at a point where the cost of construction of a bridge, or of the approaches to it, will be more favorable than the one in which it is intersected by the right line.

Changes from the direction of the right line may also be favorable for the purpose of avoiding the intersection of secondary water-courses; of gaining a better soil for the roadway; of giving a better exposure of its surface to the sun and wind; or of procuring better materials for the road-covering.

By a careful comparison of the advantages presented by these different features, the engineer will be enabled to decide how far the general direction of the right line may be departed from with advantage to the location. By choosing a more sinuous course the length of the line will often not be increased to any very considerable degree, while the cost of construction may be greatly reduced, either in obtaining more favorable gradients, or in lessening the amount of excavation and embankment.

640. When the points of arrival and departure are upon different levels, as is usually the case, it will seldom be practicable to connect them by a continual ascent. The most that can be done will be to cross the dividing ridges at their lowest points, and to avoid, as far as practicable, the intersection of considerable secondary valleys which might require any considerable ascent on one side and descent on the other.

641. The gradients upon common roads will depend upon the kind of material used for the road-covering, and upon the state in which the road-surface is kept. The gradient in all cases should be less than the angle of repose, or of that inclination of the axis of the road in which the ordinary vehicles for transportation would remain at a state of rest, or, if placed in motion, would descend by the action of gravity with uniform velocity.

The gradients corresponding to the angle of repose have been ascertained by experiments made upon the various road-coverings in ordinary use, by allowing a vehicle to descend along a road of variable inclination until it was brought to a state of rest by the retarding force of friction; also, by ascertaining the amount of force, termed the force of traction, requisite to put in motion a vehicle with a given load on a level road.

The following are the results of experiments made by Mr. Macneill, in England, to determine the force of traction for one ton upon level roads.

No. 1. Good pavement, the force of traction is

2. Broken stone surface laid on an old flint road 66 3. Gravel road

33 lbs.