The self-acting module, of which the details are

given in sectional elevation, Fig. 50 and in plan, Fig. 51, is that used on the Marseilles Canal, and differs en

tirely in principle from the one proposed for the Isabella II. Canal, described above, p. 153, in which it is arranged that as the head of water increases the outlet is diminished, and vice versa. In the module now considered the outlet remains always the same, and at the same distance below the surface of the water, with which it rises or falls, being attached to floats. It will be seen by the engravings, that at the bottom of a masonry cistern connected with the canal,-the bent arrows show the entry of the water, there is a circular orifice, c, Fig. 51, into which is accurately fitted, by a water-tight collar, an iron

cylinder, b, open at each end. This cylinder hangs by the rod n to a wooden bar, f, f, supported by two floats, g, g, on the surface of the water, and slips freely up and down in its collar. By means of a screw at h the distance of the upper edge of the cylinder from the bar, and, consequently, from the water's surface, is fixed, and that

being done so as to give the required discharge, it is never altered. The module is contained in a small locked house. The water below the orifice, after having passed over the circular edge, goes direct to the irrigation channel, as shown by the external bent arrow. It does not require the same loss of level as that of Isabella II. ; but it is difficult to believe that the iron cylinder, however nicely adjusted, can always work true, and without either friction or considerable leakage. The silt is observed to collect about this module in considerable quantities. It is, however, stated to satisfy both the canal engineers and the irrigators who pay for the water, so that its working can hardly be very inaccurate, and thus a great object has been obtained. The chief dimensions have been given in feet and decimals.

The water module designed for the Henares Canal in Spain by the eminent hydraulic engineer, J. F. Bateman, is thus described by Lieut. C. C. S. Moncrieff in the work already quoted :-About 4 miles from the weir at the head of the canal is the first module for an irrigating channel, branching off from the main canal. Each of these channels is to have a discharge not exceeding 6.22 cb. ft. per second (= 176 litres, nearly 10 cb. metres per minute). The modules are of a rather expensive construction, costing £60 each, and will doubtless do what is intended very efficiently. The self-acting principle has not been tried at all; but the regulation is effected, as shown in Figs. 52 to 56, by means of a very neatly fitting cast-iron sluice, raised and depressed by a screw, and admitting the water into a masonry chamber, out of which it escapes over a bevilled iron edge.

The guard in charge has orders to keep the level of the water to a certain height, denoted by a gauge, in this chamber. To effect this he opens or shuts the sluice according to the fall or rise in the canal. The water passes the

sluice with of course a good deal of boiling action, which is completely stopped by a species of masonry grating, M, Figs. 54 and 55, built across the chamber, dividing it into eight passages, each

SECTION ON E.F.
9.18

the

the lower side of this partition the water is perfectly still, and drops gently over the iron edge. The opening of the sluice is 1.97 ft. x 1.97 ft. (= 0.60 × 0.60 metre); length of the iron edge is 6.56 ft. (= 2 metres). The depth then of the film of water passing over it when it discharges 6.11 cb. ft. per second, will be 5.10 inches. Fig. 53 gives

an elevation of the

cast-iron sluice on the

ELEVATION ON A.B.

Figs. 52 and 53.

side of the main canal, with the actuating screw, wing

walls, &c., and it is shown in vertical section at N, Fig.

B

C

Fig. 55.

its masonry grating, is shown at M in the longitudinal section, Fig. 54, and also in the transverse section through EF, Fig. 52, and again, in the general plan at M.

The cast-iron overfall is shown in section, at an enlarged scale, in Fig. 56, and the upper step on which the water tumbles, all three of which are shown in Figs. 54 and 55. So long as perfect reliance can be placed on the honesty of the guards, the distribution will be effected with great regularity.

1.32

1.37

66.

Fig. 56.

The action of the instrument for measuring the velocity of rivers, called Pitot's tube, helps to explain some of the subsequent Practical Examples. It is shown in Fig. 57, and consists of a glass tube bent at a right angle, and having at one end a bellmouth which is immersed in the current horizontally, and turned so as to face up stream, the other end being

M