elevated surface of the dammed-back water will during the ebb as gradually fall to the low-water level.

If, however, the river is confined between high embankments at its mouth, and if the channel upwards is of unequal breadth, being narrow in some places, and spread out into basins of great width in others, so as to form inland seas or lakes during high water, then "races" (as they are sometimes termed on old plans and in old deeds) will be found in the narrow parts between them. The force of the stream in such places is not easily calculated, or embankments defended from the impetuosity of the current during spring-tides, when a more than ordinary rapid fall of the ocean in front is attended by a heavy inland flood behind, or vice versa, when an extra high tide is accompanied with a low level of the river, so that there is little or no counterbalancing action to the upward current, by the damming process of the downward one.

3. The velocity of the river as determined by its tidal length.

The velocity of a tidal river at low water is inversely as the length of the tidal part; or, the shorter the distance between the mouth of a tidal river and the highest point to which the tide reaches, the greater the velocity of the river at low water; and, vice versa, the greater the distance the less the velocity.

By straightening a meandering tidal river its velocity at low water will be increased, because its course will be shortened.

The practical data upon which the above two rules are founded may be given thus. If we assume the difference of the depth of the river at its mouth between low water and high water to be 12 feet, then this is the fall of the whole tidal portion of the river included in the survey. If we next assume that the length of the tidal part is twelve miles, that the ground is of equal quality and the inclination uniform throughout its length, it will give a fall of one foot per mile. But if the length is twenty-four miles, and the other conditions the same, then the fall per mile will only be six inches; consequently the velocity in the latter example will only be that due to six inches, or one half the former.

The formation of bars in the beds of rivers is produced by a decrease of velocity, the sand and gravel being deposited when the force of the current is so reduced as not to be sufficient to carry them farther. Bars across the mouths of rivers are formed in the same way, or jointly by the action of the sea.

The extra depth of the tidal part of a river which gradually increases towards the sea, when the bottom soil is of uniform quality, is produced, partly by the accelerated velocity of the water during

the ebbing of the tide; partly by the scooping-out process of opposing currents during the flow of the tide; and partly by the action of the waves towards the sea in stormy weather.

If the ground forming the bed of the river is not composed of materials of uniform quality, the inclination of the bottom will not be uniform throughout its length.

Bars formed in the bed of the river require to be shown both by longitudinal and transverse sections; inequalities in the bottom, produced by different gradients, only in the former, unless special circumstances call for a transverse representation. Bars beyond the mouth of the river seawards are usually shown upon the plan, the depth of water over them being represented by figures. All bars, both at sea and in the channel of the river, if dry at low water, require to be surveyed and shown upon the plan as dry land; where rock crops out in the bottom, it should be indicated both on plan and section, and also any special diversity of soil, as a change from clay to sand.

4. New channel and embankments.

A new channel for a river may be occasioned either by changing its outfall, or straightening the course of the old one, and both (it may be presumed) are included in the survey.

A new outfall.

In surveying ground for a new outfall, the inclination of the bottom of the sea and the quality of the ground require, in the vast majority of cases, to be carefully examined for a considerable distance out seawards, before the depth and breadth of the mouth of the new channel can be determined, according to the directions given under the second head of this example. The exposure of the new outfall also requires to be attended to; for if the one side of the channel is more exposed than the other, a greater slope must be given both to it and the embankment, so as to make the return action of the water down the inclined plane defend it, as formerly shown, during a heavy storm.

In a few exceptional cases the new outfall may be made over a rocky bottom at various levels between high and low water, in which the breadth of the river from its mouth upwards will be determined accordingly. Thus, if the new channel is made to intercept the water from the elevated grounds on one side of the low lands, and to discharge itself over a rocky bottom on a level with high water, then the new river will cease to be a tidal one and to be subject to tidal action; consequently its breadth will be determined by its

inclination and volume of fresh water, as in the next example. If, however, the rock is on a level with low water, then the breadth of the river will depend upon the nature of the ground forming the sides of the channel, and upon the exposure of its mouth to the waves of the sea during storms; and for intermediate levels the rock will defend the channel from tidal action according to its height, tidal action being inversely as the height of the rocky bottom above low water level.

Straightening a river.

In the language of practice, straightening a river more frequently means reducing the number of curves and right lines than the formation of one straight channel.

In every survey, therefore, the direction of a new channel for a river has to be determined on the spot.

If the object of a new channel is to effect a more efficient state of drainage, then the nearer its direction approaches a right line, the shorter will be the whole length of its course, and the greater the fall gained at low water for any given distance.

By such means the length of the tidal portion of some rivers in the United Kingdom may be reduced to one half-of others to one third, and of one or two exceptions to one fourth of their present length. In such cases the increase of fall gained, and also the area of land reclaimed from the river, may be easily calculated.

The directions for the capacity of the straight portion of a new channel are the same as those given for the survey of the longitudinal and transverse sections of the old.

If a new channel is to be opened between two bends of the old so as to cut off one intervening bend, the best line of direction will be that of tangents to the two curves.

In straightening a river according to this rule the radius of any curve may be easily increased, so as to change the angularity of the current when it strikes the concave bank at too acute an angle. This may be done, for example, by extending the tangential direction of the new channel outwards, the required distance from both the old bends, so that the two station poles at the two extremities of the straight portion of the new channel shall be in the two curves at the two points where the two radii touch the new line of channel, so as to form with it right angles.

In the field this is done by going backwards until the whole of the curve on one hand and new channel on the other shall appear on the river side of the station poles; and the centre of the circle at each curve is the point where two lines ranging from station poles

at the two extremities of the curve intersect each other, the one line forming a right angle with the new channel, upwards or downwards as the case may be, and the other a right angle with the direction of the old channel, downwards or upwards as the case may be.

The less the radius of any curve, the less must be the inclination of the slope of the channel and embankment on the concave side, to defend it from the current, and, vice versa, on the opposite or convex side to maintain the capacity of the river required by tidal action. A transverse section of a curve of a river is thus different from that of the straight portion of the channel.

5. Methods for counteracting tidal action.

One method for obviating the destructive action of tides in tidal rivers has already been given, viz., by changing the outfall to a rocky bottom.

A second method is by a tide-lock, or several tide-locks, if the volume of water is large. Such may be constructed either at the mouth of the river, or in the first narrow place above it, if preferable.

A third method may consist of a tide-lock for shipping at some distant point communicating with the river by means of a canal, the channel of the river itself being defended by a series of waterfalls from the high to the low water level, constructed on the principle of a salmon ladder, and which might serve that purpose.

A fourth method consists of floodgates or sluices, in number according to the size of the river, across its mouth or some more convenient place above it.

These several methods are now in operation, and hence call for surveys.

The details of the practice are similar to those of canal-surveying, and will be given under Example VI.

6. Influx of tributary rivers.

The survey of a tidal river generally includes that of its tributaries to a distance as far up from their mouths as contemplated improvements extend.

Where lands have been reclaimed, or are proposed to be so, at the mouth of a tidal river, as in the example, the first two tributaries surveyed are those that previously discharged, or now discharge, the drainage waters of the more elevated grounds through the unreclaimed land into the sea, which require to be diverted from their

old channels and turned into the river by two new courses—one on each side the main stream.

Where old embankments exist at the inland boundary of the newlyreclaimed land, they may probably be turned to profitable account by forming one of the embankments of the first tributary, if the upland drainage-water cannot be intercepted at a higher level.

In those cases where no land has previously been reclaimed, and where embankments do not exist, the high-water levels of springtides often determine the course of the two first tributaries for intercepting the drainage-water of the elevated grounds, and discharging it into the main river by opposite channels.

These are questions that always require to be determined on the spot by taking the levels of high water at the necessary station poles, and the other station-levels on more elevated ground. They are seldom surrounded with much practical difficulty; and, as the formation of the two first intercepting tributaries form the initiatory work of the contractor in the reclaiming of the lands lying between them and the ocean, this division of the survey should be the first finished, as the plans or copies of them may be demanded before the others.

The waters of a tributary and main river should unite with equal velocities, so as not to form bars across either channel, or in any way way disturb the uniform flow of the principal stream.

Where a tidal river runs through a narrow valley, tributaries very frequently enter at too great a velocity; but such can easily be reduced by means of a waterfall, or series of waterfalls, as in the case of mountain streams in Example V.

The longitudinal and transverse sections of tributary rivers are similar to those of the principal, so that the directions given relative to the latter are applicable to the former.

EXAMPLE III.

GENERAL DIRECTIONS FOR

THE HORIZONTAL SURVEY OF LANDS RECLAIMED UNDER THE FIRST AND SECOND EXAMPLES.

When a large area of land has been recovered from the sea and a tidal river by embankments, it requires to be surveyed and laid out into fields and farms for agricultural purposes. And if it belongs to different proprietors, as is frequently, if not generally, the case, when the reclaimed lands extend to both sides of the river, the whole will have to be apportioned according to the several proprietary rights involved.