IUtricr-/cz.c/ Gans-cs.—Further, as no hydraulic work can be under taken without proper consideration of the various levels peculiar to the stream, it is customary (in the case of all streams 'of any importance) to place at points convenient for observation gauges to measure the levels of the water. These are usually graduated from below upward, the gradua tions being made with the greatest acCuracy and marked very plainly, and from these observations of the levels of the water are taken daily and duly recorded in a book kept for the purpose. The zero-point of the gauge is usually taken somewhat below the lowest water-level.
Profile work of obtaining the data for mapping the longitudinal profile and the numerous cross-profiles of the stream is also of much importance. To determine the first, a line of stakes is planted, which follow the windings of the river, and these are chained and levelled. The slope of the water-surface is determined with great care. For the cross profile, measurements of the depth at various points are necessary. These measurements are made with the measuring-rod, or, in the case of great depths, with the plummet. The cross-sections are referred, both as to position and as to direction, to the above-named line of stakes.
Uclocity of Flow: velocity of flow of a stream will depend npon the inclination of the surface. In a portion of the stream where the cross-section for a good distance is tolerably uniform, the velocity of the current will always be uniform and may readily be measured with suitable instruments. For this purpose floats of various kinds (spheres of wood or of wax, or long rods weighted at one end to keep them in a vertical position) are placed in the current, and the time required by them to pass between two previously-fixed cross-sections is noted. Pitot's tube, which is used for this purpose, is an open tube shaped like the letter L. This is placed in the water in such a manner that the open end of the submerged leg is held facing the current, and the velocity of flow is measured by the height above the surface to which the water rises in the vertical leg. The more rapid the flow, the higher will the water rise in this. The Woltinann tachometer is a wheel-meter which may be attached to a rod and immersed in the stream to any desired depth. The rotation of the wheel is communicated by suitably-disposed gearing to an index, which registers the number of rotations, and from this the velocity of flow may readily be calculated. The registering mechanism may be thrown in or out of gear by the observer at the surface. When the velocity of flow at various points and depths of the cross-section has been observed, its mean value may be obtained by properly averaging these observations, and by multiplying the resulting mean value by the area of the cross-section the volume of water passing, through the cross-section in a given time may be ascertained. The law of the relation between depth and velocity of flow
is not definitely known. Instead of making direct measurements, the mean velocity of flow of a stream may be determined approximately by calcula tion when the fall, the mean area of cross-section, and the character of the bed of the stream are known.
11701-kS: FCISCilleS.—In the construction of the works required for the rigime of a stream which take the form of embankments, training-walls, jetties, dams, levees, etc., use is made of stone, sand, and bundles of brush wood, called fascines (pi. 57, fig. 1), the latter being formed of successive superposed layers of fascine-mattresses sunk at a suitable angle, each laver being covered with stone, sand, gravel, etc., before the succeeding laver is sunk upon it. The layers of faseines are first put together floating on the surface. The weighting- of a layer causes it to sink, and in so doing to turn upon a horizontal axis; and the junction which it forms with the previous layer is thus made in a manner which prevents the ballast from sliding off (fig. 6, right). The definitive slope of the fascine-bed is attained only after a time. The uniting of the faschies into compact masses is effected by means of plaiting, the same with willow whiles attached to strong- stakes passing through the bundles (Jigs. 4, 5). To unite a mat tress of this description with the river-bank, siiitable cuts are made, into which the ends of the layers come to lie when sunk to place. The project ing portions of the mattress which are particularly exposed to the action of the current are protected with stones. The top is plaited around the edges, the spaces between being- filled in with earth. To consolidate more firmly the materials, bushes are cultivated upon the crown of the construc tion, whose surface is commonly raised only about a foot above the water surface, that the veg-etation may thrive; or the surface may be protected by a pavement. Larger constructions of this nature may be built conveniently on the shore. These are usually formed of a number of layers of fascines, which are laid crosswise one upon another, and are secured by stakes and bound together by means of ropes or withes. These masses are floated to the desired spot, and sunk by weighting with earth, stone, etc. Fig-ures 2 and 3 exhibit modified forms of these constructions.