length of the lines between the station-poles in the two boundary lines will depend upon that of the base line. In the field the significant practical rule, the best of three, is generally given, i.e., measure the base line three times and take the average.

The bearings are taken as in ordinary surveys with the theodolite, and directions for such will be found in Sect. VI. Part IX. and p. 385.

The stations and lines which they form should be shown upon the plan, as represented in the plan of the Preliminary Survey for a projected Railway, Part X. Sect. I. And the two boundary lines may be represented in a similar way to what the two lines indicating the Limits of Deviation of the Parliamentary Survey are in Plate XIII.

If the surveyor undertakes to lay down a line representing the centre of the embankment, it may be determined as in Sect. I. Part X., and shown between the exterior and interior boundary lines upon the plan, as the line of railway is represented in Plate XIII. between the two lines indicating the Limits of Deviation.

The object of thus showing the working details of the survey, both horizontal and vertical, is to enable engineers, contractors, or their draughtsmen to take off working plans more readily for the execution of the work of embanking, &c., and for this purpose doubles of the plans and sections are generally ordered when surveys are extensive.

Where several landowners are interested they may order an approximate estimate of their respective shares of the land to be reclaimed, which can easily be taken and shown upon the plan, by rules previously given in the work. But the final work of apportioning shares should be left to a more complete survey when the embankments are finished, and when the lands recovered from the sea and the river can be laid out into fields, and the lines accurately measured with the chain. (See Ex. III.)

The position of sluices, syphons, or pumping stations, and existing embankments, require to be taken and shown upon the plan, and also the position of watercourses necessary for the drainage of adjoining lands that lie above the level of the top of the embank

ments.

The general rule for the latter is, that no water should be allowed to descend from a higher level into the watercourses of the lands to be reclaimed that can otherwise be drained off to the sea or river.

Directions for Longitudinal and Transverse Sections for

Embankments.

When the survey includes longitudinal and transverse sections of the embankment, it is more accurately and easily done to show them upon a separate plan by themselves, than to crowd them into the preliminary one, along with those previously given.

This part of the survey includes the ground from whence materials are to be obtained for the formation of the embankment; and the quantity of materials will depend upon the dimensions of the embankment, quality of soil, and exposure to the sea.

The exceptions are few where the foundation of an embankment can be safely laid upon the surface of the ground to be reclaimed. Even in those cases where it has to be run through a shallow arm of the sea, according to the plan pursued in many cases in Holland, and also in this country, a proper foundation should afterwards be dug out inside of sufficient breadth to contain a body of clay that will prevent the water outside from finding its way by any porous strata to the interior, and thus ultimately endangering the safety of the whole area of land reclaimed.

As a general rule, sea embankments should be formed of inorganic materials, as clay or clayey gravel, and the nearest ground containing such, if at command, is generally that included in the survey. With the means of railway conveyance now at command, contractors enjoy many facilities for getting an ample supply of such materials that were not within the reach of their predecessors, so that if the surveyor finds a choice, the best quality may be preferable, although situated at a greater distance.

The principal reasons why vegetable and other organic matter should be excluded from the interior of embankments are as follows:

:

1. All organic matter of the kind in question is subject to decomposition. If, therefore, an embankment, or any portion of it extend ing from the outside to the inside, is formed of materials that contain a large percentage by bulk of such, this percentage may in the course of time be dissipated in the atmosphere, and the inorganic portion reduced to an open porous body approaching in character as if formed of pure sand or gravel, through which the sea-water will ooze, and eventually flow.

2. Embankments which contain a large percentage of vegetable matter are liable to destruction from vermin of a very multifarious and indefinable character, as rats, moles, mice, worms, and innumerable kinds of insects, some of which are so small as to be invisible

to the naked eye. This motley brood live upon each other and the decaying vegetable matter as their normal food: the organic portion of the embankment is thus literally consumed, while the inorganic part that remains is so burrowed as to resemble in some measure a sponge, through which the sea water will ooze, eventually effecting a breach, as in the previous case.

3. A third reason may consist of a combination of the first and second, and in practice this is perhaps what is generally experienced, the depredation of insects and other animals, together with the action of their excreta and putrefying remains, hastening the chemical changes that destroy the stability of the embankment.

When old embankments that have to remain are included in the survey, they should be carefully examined in order to have destructive causes of the above kinds, and the extent of injury done, faithfully delineated. For this purpose a good microscope is essentially necessary; and if it is discovered that the embankment is giving way, it may be advisable to recommend the employment of men to whose professional labours these branches of science belong. The amount or area of damage done will, however, require to be examined by the surveyor, and shown upon the plan and section of the old embankment.

EXAMPLE II.

DIRECTIONS FOR THE SURVEY OF A TIDAL RIVER.

The present survey may be presumed to contemplate improvements extending from the mouth of a tidal river upwards to the highest point where the tide flows. The field operations lie between Examples I. and IV., and are chiefly sectional in character, embracing longitudinal and transverse sections of the channel and its embankments.

Many of the directions given in the last example relative to the ranging of the station poles, the taking of levels at high water, and the survey of the horizontal and vertical planes for the preliminary plan and sections, are applicable to the present, and therefore need not be repeated.

The chief points requiring notice are as follows:-First, the breadth of the river as determined by its tributaries. Second, the breadth of the river as determined by tidal action. Third, the velocity of the river as determined by its tidal length. Fourth, new channel and embankments. Fifth, methods for counteracting tidal action. Sixth, influx of tributary rivers.

1. The breadth of a river as determined by its tributaries. The increase required in the breadth of a river to contain the waters of tributary streams, so as to maintain its velocity at low water and prevent damming back above the point of junction, is directly as the volumes of water which the tributaries discharge.

If, for example, a tributary river discharges a volume of water equal to that flowing in the tidal river, then the breadth of channel required by the latter below the point of junction will be double that above where the two rivers unite. Or the breadth of the united rivers, as it requires to be shown upon the plan and transverse section, is equal to the sum of the breadths of the two rivers before their junction, their velocities and volumes of water being equal.

Had the tidal river contained twice the volume of water it received from its tributary, then the increase of breadth required would have been one-third; and for any other increase of volume, greater or less, a corresponding increase in the breadth of the channel at low water is necessary, to preserve the natural velocity of the tidal river below the point of junction, and to prevent the damming back process above this point (allowing no increase of breadth for friction, which must always be determined on the spot).

In applying the above data either to the laying out of a new channel for the tidal part of a river, or in straightening or effecting any improvement of the old one, it must be borne in mind that an increase in the depth will not compensate for breadth. The rule is founded upon an established law that cannot be violated without consequential losses being sustained, for a greater velocity effected by an increased altitude of the surface-level of the water, involves the damming back process above the point where this superelevation of the surface-water takes place; and when the increase of depth is effected by digging a deeper channel, the practice is abortive, amounting to a standing pool below the natural inclination of the bed of the river, so that no increase of velocity is obtained to convey the extra volume of water received from the tributary.

2. The breadth of a river as determined by tidal action.

The action of a tidal river upon the sides or slopes of its channel and embankments, and also upon its bottom, is directly as its depth, the quality of the ground being equal or uniform.

This rule is based upon the established data at sea, viz., the deeper the ocean the higher the waves and the greater the force with which they strike the vessel or lash the shore, and the heavier the amount of damage done during a storm.

On a sandy, gravelly, clayey, or muddy sea-shore, the counteracting force of the return water down the inclined slope formed by the flowing and ebbing tide is a safer defence to the slope than the more solid materials of its structure.

Under differences of geological strata the coarser the quality of the gravel or sand, the more obtuse is the angle of the slope of the sea-beach, and vice versa.

The same practical data will be found exemplified in beds of rivers that have been scooped out by the natural action of the

water.

The above data may be seen instructively illustrated on a seabeach during a storm, or channel of a river during heavy floods; and both examples should be examined on shores of different qualities of soil, to obtain the invaluable information they afford.

The length of the transverse section representing the breadth of a tidal river at its mouth will therefore be as the height of the highwater level line above the ground line or bottom of the channel; but inversely as the angle of depression which the ground line makes with the horizon or high-water level.

As the extra depth of the river above its mouth produced by tidal action gradually decreases until it ceases to exist, it consequently follows that the extra breadth required for tidal action will also decrease in a corresponding manner, and finally cease to exist at the point where the tide ceases to flow.

This gradual decrease of breadth requires to be carefully ascertained in the survey and delineated upon the plan, and also shown by the transverse sections of the river, at its mouth and at the junctions of the tributary streams and upper end of the survey where tidal action terminates.

The general doctrines taught in the preceding directions have reference to the uniform retarded and accelerated velocity of the river during the flow and ebb of the tide, so essentially necessary to the safety of its channel and embankments. Thus during the flow of the tide the velocity of the river will be retarded, and the direction of the current at its mouth and upwards gradually reversed, owing to the damming back process that takes place by the rising of the surface-level of the ocean above that of the water in the river. But when the ebb commences the river will again begin to flow to the ocean with an accelerated velocity. Now, if the above rules have been properly carried out, then the retarding and accelerating processes will be so uniform as hardly to be perceptible to the eye of the observer, the downward current of the river being gradually counteracted by the upward flow of the tide, while the