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in the manifestation of the diurnal inequality may easily be accounted for, by supposing the two waves, having been originally formed in different latitudes, to take somewhat different routes. One being perhaps a little more easterly than the other, may find a less obstructed path in the ocean than that which is more directed along the shore; it may thus arrive sooner, and be smaller when it arrives at Bassadore. In its route to the Gulf of Cambay, it may meet with circumstances which retard it and increase its dimensions, in an equal degree with those which by a different route have affected the other.

This is a mere conjecture, which more perfect knowledge may refute or confirm, but we mention it to show how terrestrial causes may affect differently tides formed on different parts of the earth's surface. The tide is here also two days old.

The tide wave flows up the Persian Gulf, and at the head there is a rise of 7] feet.

The progress of the wave along the southern coast of Arabia is generally to the westward, but it is stated to be irregular ; probably its continuous movement is interfered with by meeting a more direct portion of the wave travelling from the south-east. The general range is from 6 to 8 feet; the time, at new and full moon eight to ten o'clock, and at the Straits of Babel. mandel twelve o'clock.

There are but few observations of the tide in the Red Sea, and the range is small. It is probable that there is a peculiar movement of the wave, of the exact nature of which we cannot judge without more observations. The time of high water at Suez appears to be nearly the same as at the Loheia River, a little within the straits. It is scarcely likely that the wave travels over this space in twelve hours, and the small rise and irregular time near Mecca would seem to indicate a movement somewhat similar to that which we find on our own coasts, and which we shall describe in the sequel.

The tide wave appears to fall directly upon the east coast of Africa, making high water generally along the coast at nearly the same time, between one and two o'clock by Greenwich time; except in some cases, where either the wave is retarded by the conformation of the ocean bed, or by some peculiar undulation. The range is pretty large in the Mozambique Channel and for some distance to the northward, being from 10 to 12 feet.

We have been compelled to extend our remarks on the subject of the tides much beyond what was our intention at first, but we have found it necessary, in order to impress upon our readers the importance of all the observations they may have to make on the subject, and at the same time to afford some clue as to that which may be required, either in the tidal survey of two or three spots at some distance from each other, or along the whole length of a line of coast, or up a tidal river or estuary.

In the United Kingdom, and in Europe generally, there is so much to be obtained from public documents, such, for instance, as the Admiralty Charts, that the surveyor has only as it were to fill up gaps occasioned by the want of details ; but where he has also to find out the leading features as well as the details as to the tides and currents, the work is often laborious, and must necessarily extend over a considerable period of time.

One, two, or a much greater number of tide gauges may be

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required, not only to ascertain the total rise and fall of the tide at different places, but the intermediate heights between high and low water, and often particularly half tide by time. One step that may be first taken, is a careful set of levels

, to different spots near low-water mark, and to ascertain by observation the total ranges of the tide at each place, both as regards springs and neaps; we shall then know where the gauges will be required.

In the surveys of tidal rivers, and of estuaries, the numerous bends, contractions, and expansions of the shores, and often of projecting bluffs under water, or any marked changes in the bed, either longitudinally or transversely, will all be found to affect

ess the parallelism of the tidal wave; and he again sets of tide gauges, all referring to one carefully ascertained level will be required, for the bulks

or velocities of the tides will be interfered with at such spots, and consequently the parallelism of the wave; the greater the rise of the tide, and the more rapid the currents, the more numerous the tide stations will often have to be.

The tide-staff erected at each station will require to be from six to nine inches wide, and steadied in a strong frame for its support and safety, unless it can be fixed against some rock or bank, having its foot reaching to low-water mark; the foot of the staff should be sunk some two or three feet below the lowest reputed low-water mark; in length the staff should be some four or five feet higher than the highest reputed spring-tide; it should be painted white and divided into feet, and the numbers of these should be shown in large bold figures, that they may be seen at some distance; each foot may be subdivided by lines into halves and quarters, but no figures shown for inches.

On a long shelving bank, or where the rise of the tide is great, the staff may be divided into short equal lengths, and carried along, as it were, in steps, the foot of each length being carefully adjusted by levelling, to correspond with the top of the length of staff below. A careful, trustworthy man should be appointed to each staff, and his observations should be taken every ten minutes; he should, therefore, be provided with a watch. With regard to high and low water, he should not only watch the highest rise and greatest fall, but also the time of the change of the current. A set of tidal observations, to be of any value, must extend over a considerable period of time, and will of course be expensive; unless great care be observed, numerous errors will creep in; the whole may therefore be invalidated, and so much money thrown away.

The staff above described will be sufficient, where it can be so

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placed as to be protected from the undulations of waves; but for harbours and coast work, it will be necessary to shelter the staff by means of a piece of stout glass, between which and the staff the water may rise and fall without being affected by the undulation of the wave. A large hollow copper ball has been used, rising and falling within a casing; to the ball is attached a chain, working by a kind of clock-work on a dial-plate.

For measuring the velocities of currents moving at the rate of above two miles an hour, Massey's patent log will give all that is required. Below this velocity, Elliott's Current Meter, which we shall describe in another place, is preferable.

It need not be added that, in taking the velocity of a current, its direction must at the same time be observed, as also the spot from whence the observation has been made. To perform this, ample instructions have already been given.

An illustration has been given of the method adopted to show the rise and fall of the tides on paper. On the horizontal line equal distances denote equal times, and the reverse; they may represent hours, days or months, or fractions of them. The rise or fall is plotted on the vertical lines corresponding to relative time.

CHAPTER XIV.

Survey of Gathering Grounds for Water Supply-General

Observations.Gathering Grounds.Objects of the Survey.

-Rainfall.Meun annual depth of Rain, with Notes. Observations on the Causes of Differences.-Rain Gauges ; the Gauge Glass.— Variations in depth of Rainfall. Position of Rain Gauges.Discharge froń Rivers and Streams, and from large Districts.-Stream Gauging.Theoretical Discharge. Contracted Vein. Rules for Weirs and Overfalls.Coefficients.-Initial VelocityEstablishment of Gauges.-Šurface and Mean Velocities. -Current Meters.

PUMPING from wells sunk to a certain depth, from rivers with or without storage reservoirs, and a system of gravitation from reservoirs supplied from extensive gathering grounds; these singly, or combined, and modified in various ways, are the principal methods adopted by engineers for supplying towns with water; often also for canals, and on a smaller scale they are likewise adopted to obtain a supply for mills and extensive manufactories where large quantities of water are required.

With very few exceptions, more particularly with regard to wells and reservoirs supplied from gathering grounds, the surveying engineer has not only to ascertain the quantity of water that

may be obtained from the above sources; he has also to define the quantity of water already appropriated by existing interests, how they will be affected by the quantity proposed to be withdrawn, and how compensation can be made in the best manner for all parties. Wells are generally sunk through some overlying impermeable stratum, as clay or rock, down to some water-bearing bed from which the fluid will rise; or rather the well is sunk to a certain depth to receive the uprising water, which is then tapped by means of a bore-hole, which may be six or even eighteen inches in diameter, as may be required, and deep enough to reach from the bottom of the well or store to the water-bed; the only way to ascertain the quantity of water obtainable by this means is by direct experiment.

A gathering ground, properly so called, is any area of land the

rain-fall on which may be collected ; it is always more or less affected by certain causes, such as geological formation, the dip of the strata, the altitude, geographical position, the character of surface-soil, or of the substratum, &c.

The storing of water from a gathering ground into a reservoir, and sometimes a chain of reservoirs, is effected in the most economical manner by embanking across one or several valleys; it will therefore be readily seen that in most cases the land survey required will be more or less extensive, for all the valleys leading to that to be embanked, will have to be shown on the plan, and very often also those separated from it by hills on one or both sides; a scale of from one to three inches to one mile is sufficient for this purpose, and in England the Ordnance maps are generally used for the purpose of showing the lines of watershed, their areas, and the position of reservoirs; the lines of watershed show the boundaries of the lands on which rain falling, may by gravitation be expected to flow towards the streams and rivers by which the waters of the valleys are discharged.

It will not be difficult to perceive why the parallel valleys have often to be surveyed, when it is considered that a short heading driven through the intermediate hills into such valleys may often be the means of leading considerable quantities of water into the reservoir ; similarly, feeders may be laid to effect a like object by bringing rain or spring waters into the reservoir valley, at a point much higher than that they would otherwise flow to.

High lands are much more favourable to the system of storage from gathering grounds than a low lying country, where not only the rainfall is smaller, but where also the ground is much less favourable for the construction of reservoirs. The consequence of this is, that whilst low flat districts are mostly supplied by pumping from rivers, towns in more elevated regions are supplied by gravitation from reservoirs constructed in valleys collecting water from gathering grounds.

The dip of the strata, in many hills surrounding a valley, is often such, that the rainfall due to the reservoir valley may be appropriated by another, by reason of the strata on one or both sides dipping in opposite directions, or away from the valley where it is proposed to impound the water; for evidently in such a case the percolating water may be carried away in an opposite direction, unless caught or brought back in the manner above suggested. Or we may have the reverse of this, by reason of the geological stratification being such, that a much greater quantity of water is brought within the drainage area, than that due from mere rainfall; in this latter case, however, the water will make its appearance in the shape of one or more

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