these cases locks must be so arranged on each side, where the canal enters the stream, that boats may pass from the one to the other under all circumstances of difference of level between the two. The locks and the portions of the canal which join the stream must be secured against damage from freshets by suitable embankments; and, when the summer water of the stream is so low that the navigation would be impeded, a dam across the stream will be requisite to secure an adequate depth of water during this epoch.

729. Canal Bridges. Bridges for roads over a canal, termed canal-bridges, are constructed like other structures of the same kind. In planning them the engineer should endeavor to give sufficient height to the bridge to prevent those accidents, of but too frequent occurrence, from persons standing upright on the deck of the passage-boat while passing under a bridge.

730. Waste-Wier. Waste-wiers must be made along the levels to let off the surplus water. The best position for them is at points where they can discharge into natural water courses. The best arrangement for a waste-wier is to make a cut through the side of the canal to a level with the bottom of it, so that, in case of necessity, the waste-wier may also serve for draining the level. The sides and bottom of the cut must be faced with masonry, and have grooves left in them to receive stop-plank, or a sliding gate, over which the surplus water is allowed to flow, under the usual circumstances, but which can be removed, if it be found necessary, either to let off a larger amount of water, or to drain the level completely.

731. Temporary Dams. In long levels an accident happening at any one point might cause serious injury to the navigation, besides a great loss of water. To prevent this, in some measure, the width of the canal may be diminished, at several points of a long level, to the width of a lock, and the sides, at these points, may be faced with masonry, arranged with grooves and stop-planks, to form a temporary dam for shutting off the water on either side.

732. Tide, or Guard Lock. The point at which a canal enters a river requires to be selected with judgment. Generally speaking, a bar will be found in the principal water course at, or below, the points where it receives its affluents. When the canal, therefore, follows the valley of an affluent, its outlet should be placed below the bar, to render its navigation permanently secure from obstruction. A large basin is usually formed at the outlet, for the convenience of commerce; and the entrance from this basin to the canal, or from the river to the basin, is effected by means of a lock with double gates, so arranged that a boat can be passed either way, according as the level in the one

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

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:

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 formed 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

sions have been adopted :

:

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 dimen

sions :

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:

Width of canal at top,

Width at bottom,

Depth of water,

Length of locks betwen mitre-sills,
Width of locks,

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,

Depth of water,

Width of tow-path,

Length of locks between mitre-sills,

Width of locks at top,

Width of locks at bottom,

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.

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.