way and the foot of the interior slope of the tow-path. The height of the tow-path is 6 feet above the berm. By increasing the depth of water in the canal to 10 feet, the water-line at top can be increased to 150 feet.

The dimensions of the Erie canal as enlarged are:

70

Width of canal at top, with bench walls.... 81 feet.
Width of canal at top, without bench walls. 75
Width of canal at water surface......
Width of canal at bottom, with bench walls. 42
Width of canal at bottom, without bench

walls....

521"
7

"41 in.

66

Length of lock (between mitre-sills).......110

824. Locomotion on Canals. In early times boats were drawn or pushed along by servants or slaves. In civilized countries horses and mules have been chiefly used. A few years since several attempts were made to use steam power, by driving the boat like a propeller, and although it would do the work, yet it was mostly abandoned after a few months. The wheel created such a disturbance in the water as caused it to wash the banks and thus damage them.

A system, known as the Belgian system, has been quite extensively used in some of the European countries. It consists of a cable which passes from one end of the canal to the other, and is sunk in it. It is wound around a wheel which is at one end of the boat. Steam power is applied to turn the wheel, and, as the friction of the rope on the wheel prevents it from slipping, it will take up the cable on one side of the wheel and let it out on the other, and thus draw the boat along. One of the objections to this plan is, it requires a large amount of slack cable to accommodate a large traffic, and every boat must draw in all the slack every time it passes over the canal.

During the winter of 1870-71 the Legislature of the State of New York offered a prize of $100,000 to the party who would make an acceptable mode of applying steam for propelling canal boats on the canals, and no plan was to be considered which involved the Belgian system. The engineer in charge of this project states that in round numbers a thousand plans, coming from all parts of the world, have been presented, but up to the present time the prize has not been awarded.

CHAPTER IX.

RIVERS.

825. Natural features of Rivers. All rivers present the same natural features and phenomena, which are more or less strongly marked and diversified by the character of the region through which they flow. Taking their rise in the highlands, and gradually descending thence to some lake, or sea, their beds are modified by the nature of the soil of the valleys in which they lie, and the velocities of their currents are affected by the same cause. Near their sources, their beds are usually rocky, irregular, narrow, and steep, and their currents are rapid. Approaching their outlets, the beds become wider and more regular, the declivity less, and the current more gentle and uniform. In the upper portions of the beds, their direction is more direct, and the obstructions met with are usually of a permanent character, arising from the inequalities of the bottom. In the lower portions, the beds assume a more tortuous course, winding through their valleys, and forming those abrupt bends, termed elbows, which seem subject to no fixed laws; and here are found those obstructions, of a more changeable character, termed bars, which are caused by deposites in the bed, arising from the wear of the banks by the current.

826. The relations which are found to exist between the cross section of a river, its longitudinal slope, the nature of its bed, and its volume of water, are termed the regimen of the river. When these relations remain permanently invariable, or change insensibly with time, the river is said to have a fixed regimen.

Most rivers acquire in time a fixed regimen, although periodically, and sometimes accidentally, subject to changes from freshets caused by the melting of snow, and heavy falls of rain. These variations in the volume of water thrown into the bed cause corresponding changes in the velocity of the current, and in the form and dimensions of the bed. These changes will depend on the character of the soil, and the width of the valley. In narrow valleys, where the banks do not readily yield to the action of the current, the effects of

any variation of velocity will only be temporarily to deepen the bed. In wide valleys, where the soil of the banks is more easily worn by the current than the bottom, any increase in the volume of water will widen the bed; and if one bank yields. more than the other, an elbow will be formed, and the position of the bed will be gradually shifted towards the concave side of the elbow.

827. The formation of elbows occasions also variations in the depth and velocity of the water. The greatest depth is found at the concave side. At the straight portions which connect two elbows, the depth is found to decrease, and the velocity of the current to increase. The bottom of the bed thus presents a series of undulations, forming shallows and deep pools, with rapid and gentle currents.

828. Bars are formed at those points, where from any cause the velocity of the current receives a sudden check.. The particles suspended in the water, or borne along over the bottom of the bed by the current, are deposited at these points, and continue to accumulate, until, by the gradual filling of the bed, the water acquires sufficient velocity to bear farther on the particles that reach the bar, when the river at this point acquires and retains a fixed regimen, until disturbed by some new cause.

829. The points at which these changes of velocity usually take place, and near which bars are found, are at the junction of a river with its affluents, at those points where the bed of the river receives a considerable increase in width, at the straight portions of the bed between elbows, and at the outlet of the river to the sea. The character of the bars will depend upon that of the soil of the banks, and the velocity of the current. Generally speaking, the bars in the upper portions of the bed will be composed of particles which are larger than those by which they are formed lower down. These accumulations at the mouths of large rivers form in time extensive shallows, and great obstructions to the discharge of the water during the seasons of freshets. The river then, not finding a sufficient outlet by the ordinary channel, excavates for itself others through the most yielding parts of the deposites. In this manner are formed those features which characterize the outlets of many large rivers, and which are termed delta, after the name given to the peculiar shape of the outlets of the Nile.

830. River Improvements. There is no subject that falls within the province of the engineer's art, that presents greater difficulties and more uncertain issues than the im

provement of rivers. Ever subject to important changes in their regimen, as the regions by which they are fed are cleared of their forests and brought under cultivation, one century sees them deep, flowing with an equable current, and liable only to a gradual increase in volume during the seasons of freshets; while the next finds their beds a prey to sudden and great freshets, which leave them, after their violent passage, obstructed by ever shifting bars and elbows. Besides these revolutions brought about in the course of years, every obstruction temporarily placed in the way of the current, every attempt to guard one point from its action by any artificial means, inevitably produces some corresponding change at another, which can seldom be foreseen, and for which the remedy applied may prove but a new cause of harm. Thus, a bar removed from one point is found gradually to form lower down; one bank protected from the current's force transfers its action to the opposite one, on any increase of volume from freshets, widening the bed, and frequently giving a new direction to the channel. Owing to these ever varying causes of change, the best weighed plans of river improvement sometimes result in complete failure.

831. In forming a plan for a river improvement, the principal objects to be considered by the engineer, are, 1st. The means to be taken to protect the banks from the action of the current. 2d. Those to prevent inundations of the surrounding country. 3d. The removal of bars, elbows and other natural obstructions to navigation. 4th. The means to be resorted to for obtaining a suitable depth of water for boats, of a proper tonnage, for the trade on the river.

832. Means for protecting the banks. To protect the banks, either the velocity of the current in-shore must be decreased so as to lessen its action on the soil; or else a facing of some material sufficiently durable to resist its action must be employed. The former method may be used when the banks are low and have a gentle declivity. The simplest plan for this purpose consists either in planting such shrubbery on the declivity as will thrive near water; or by driving down short pickets and interlacing them with twigs, forming a kind of wicker-work. These constructions break the force of the current, and diminish its velocity near the shore, and thus cause the water to deposit its finer particles, which gradually fill out and strengthen the banks. If the banks are high, and are subject to cave in from the action of the current on their base, they may be either cut down to a gentle declivity, as in

the last case; or else they may receive a slope of nearly 45°, and be faced with dry stone, care being taken to secure the base by blocks of loose stone, or by a facing of brush and stone laid in alternate layers.

833. Measures against inundations. At the points in the course of a river where inundations are to be apprehended, the water-way, if practicable, should be increased; all obstructions to the free discharge of the water below the point should be removed; and dikes of earth, usually termed levées, should be raised on each side of the river. By increasing the water-way a temporary improvement only will be effected; for, except in the season of freshets, the velocity of the current at this point will be so much decreased as to form deposites, which, at some future day, may prove a cause of damage. In confining the water between levées, two methods have been tried: the one consists in leaving a water-way strictly necessary for the discharge of freshets; the other in giving the stream a wide bed. The Po in Italy and the Mississippi present examples of the former method; the effect of which in both cases has been to raise the bed of the stream so much that in many parts the water is habitually above the natural surface of the country, leaving it exposed to serious inundations should the levées give way. The other method has been tried on the Loire in France, and observation has proved that the general level of the bed has not sensibly risen for a long series of years; but it has been found that the bars, which are formed after each freshet, are shifted constantly by the next, so that when the waters have subsided to their ordinary state, the navigation is extremely intricate from this cause. Other means have been tried, such as opening new channels at the exposed points, or building dams above them to keep the water back; but they have all been found to afford only a temporary relief.

834. Elbows. The constant wear of the bank, and shifting of the channel towards the concave side of elbows, have led to various plans for removing the inconveniences which they present to navigation. The method which has been most generally tried for this purpose consists in building out dikes, termed wing-dams, from the concave side into the stream, placing them either at right angles to the thread of the current, or obliquely down stream, so as to deflect the current towards the opposite shore.

Wing-dams are usually constructed either of blocks of stone, of crib-work formed of heavy timbers filled in with broken stone, or of alternate layers of gravel and fascines.