is higher or lower than that in the other. A lock so arranged is termed a tide, or guard lock, from its uses. The position of the tail of this lock is not indifferent in all cases where it forms the outlet to the river; for were the tail placed up stream, it would be more difficult to pass in or out, than if it were down

stream.

733. The general dimensions of canals and their locks in this country and in Europe, with occasional exceptions, do not differ in any considerable degree.

English Canals. Two classes of canals are to be met with in England, differing materially in their dimensions. The following are the usual dimensions of the cross section of the largest size, and those of their locks :

Width of section at the water level, from 36 to 40 feet.
Width at bottom,

24

66

The Caledonian canal, in Scotland, which connects Loch-Eil on the Western sea with Murray Firth on the Eastern, is remarkable for its size, which will admit of the passage of frigates of the second class. The following are the principal dimensions of the cross section of the canal and its locks :—

Width of canal at the water level,

110 feet.

The side walls of the locks are built with a curved batter; they are of the uniform thickness of 6 feet, and are strengthened by counterforts, placed about 15 feet apart, which are 4 feet wide and of the same thickness. The bottom of the chamber is form ed with an inverted arch.

French Canals. In France the following uniform system has been established for the dimensions of canals and their locks

The boats adapted to these dimensions are from 105 to 108 feet long, 16 feet across the beam, and have a draught of 4 feet.

The English and French canals usually have but one tow-path, which is from 9 to 12 feet wide, and about 2 feet above the water level. The side of the tow-path embankment next to the water-way, is usually faced either with dry stone, masonry, or planks retained by short piles.

Canals of the United States and Canada. The original dimensions of the New-York Erie canal and its locks, have been generally adopted for similar works subsequently constructed in most of the other states. The dimensions of this canal and its locks are as follows:

For the enlargement of the Erie canal, the following dimen

Between the double locks a culvert is placed, which allows the water to flow from the level above the lock to the one below, when there is a surplus of water in the former.

A well, or pit, is left between the lift-wall of the lock and the cross wall which retains the earth at the head of the lock to the level of the bottom of the canal. This pit, receiving the deposite of sand and gravel brought down by the current, prevents it from obstructing the play of the gates.

On the Chesapeake and Ohio canal, the cross section of the canal below Harper's Ferry has received the following dimensions:

The Rideau canal, which connects Lake Ontario with the River Ottawa, is arranged for steam navigation. A considerable portion of this line consists of slack-water navigation, formed by connecting the natural water-courses between the outlets of the canal. The length of the locks on this canal is 134 feet between the mitre-sills, and their width 33 feet.

The Welland canal, between lakes Erie and Ontario, as originally constructed, received the following dimensions:

The canals and locks made to avoid the dangerous rapids of the St. Lawrence are in all respects among the largest in the world. The following are the dimensions of the portion of the canal and the locks between Long Sault and Cornwall :

Width of canal at top,

Width at bottom,

Length of locks between mitre-sills,

A berm of 5 feet is left on each side between the water 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.

t

RIVERS.

734. 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 causes. 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.

735. 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.

736. 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.

737. 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.

738. 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.

739. 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.

740. 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 improvement 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