The apparatus (Fig. 277 ) consists of an elbowed pipe C A B D E of galvanised iron, 'whose extremity 0 communicates with the outlet R, where it is fixed by means of a piece of rubber of peculiar form that allows the other extremity B D E to revolve around the axis K, while at the same time keeping the outlet pipe hermetically closed. This rubber, whose lower extremity is bent back like the bell of a trumpet, forms a washer against which is applied a galvanised iron ring that is fixed to the mouth of the outlet pipe by means of 6 small screws. This ring is provided with 2 studs, which engage with 2 flexible thimbles K L, affixed to the siphon by 4 rivets. These studs and thimbles, as well as the screws, are likewise galvanised. Between the branches A B D E of the pipe is soldered a sheet of galvanised iron, which forms isolaledly a receptacle or air-chamber F, that contains at its upper part a small aperture 6, that remains always open, and, at its lower part, a copper screw-plug d and a galvanised hook H. In the interior of this chamber is arranged a small leaden siphon a b c whose longer leg a passes through the bottom, where it is soldered, and whose shorter one c ends in close proximity to the bottom.
Finally, a galvanised iron chain G H, fixed at G to the bottom of the reservoir, and provided with a weight P of galvanised iron, is hooked at H to 'the siphon and allows it to rise more or less, according as it is given a greater or less length. From what precedes, it will be seen that the outlet is entirely closed, so that, in order that the water may escape, it must pass into the pipe in the direction K D B A C. This granted, let us see how the apparatus works: In measure as the water rises in the reservoir, the siphon gradually loses weight, and its extremity B D H is finally lifted by the thrust, so that the entire affair revolves upon the studs K, until the chain becomes taut. The apparatus then ceases to rise; but the water, ever continuing to rise, finally reaches the apex 6 of the smaller siphon, and through it enters the air-chamber and fills it. The equilibrium being thus broken, the siphon descends to the bottom, becomes primed, and empties the reservoir. When the level of the water in descending is at the height of the small siphon a b c, this latter, which is also primed, empties the chamber F in turn, so that, at the moment the large siphon loses its priming, the entire apparatus is in the same state that it was at first.
In short, when the water enters the reservoir, the siphon, movable upon its base, rises to the height at which it is desired that the flow shall take place. Being arrested at this point by the chain, it becomes primed and sinks, and the water escapes. When the water is exhausted, the siphon rises anew in order to again sink; and this goes on as long as the period of irrigation lasts. This apparatus, which is simple in its operation, and not very costly, is being employed with success for irrigating several meadows in the upper basin of the Allier. (Le Genie Civil.)
Siphon primed by blowing.
Self priming siphon.
(e) As well known, the general solution of the problem of storing water, a vital question for agriculture, is the following: To unite all the sources that are not utilisable, on account of their too feeble discharge, in a reservoir of appropriate dimensions which is emptied one or more times in 24 hours through a sluice of dimensions such that the water collected can be entirely distributed over the surface to be irrigated, in a relatively short time. Experience, in fact, has proved that if water is profitable distributed to profusion, it is but slightly so when it flows in a thin stream in a trench of which it wets only the banks.
Instead of having a sluice to be opened at definite intervals, it long ago occurred to some persons to make use of the ordinary siphon. It suffices, in fact, that the latter shall prime itself automatically in order to have a rapid and intermittent emptying of the reservoir. But the conditions necessary for such automatic priming are sometimes difficult to carry out. The source, in fact, must be very regular, and have a pretty large discharge, larger than that of the siphon during the short space of time in which the latter, operating at first as a waste pipe, is upon the point of priming itself. If this critical point is passed, the priming is effected and the reservoir is emptied by reason of the greater velocity that the head of water gives the liquid in the siphon.
But if the source is intermittent, irregular, or diminishes, it may happen that the siphon will no longer perform the functions of anything but a waste pipe. Priming will no longer be able to be effected, and the abrupt emptying of the reservoir will no longer take place.
In certain special cases, this state of things can be remedied by establishing a well of water for the reception of the long branch of the siphon. The overflow is thus reduced and the priming can take place.
This, in reality, is merely a palliative of a result that is so uncertain in all cases that it is usually preferred to empty the reservoir by opening a sluice at stated intervals. Hence an annoying restraint, and a very poor utilisation of the water at one's disposal.
In fact, the user generally opens the sluice in the morning and evening. Between these two intervals and -at night, if the reservoir is full, the water flows out slowly, and irrigates but a small surface.
Different means have been proposed for obtaining an automatic discharge, and especially for preventing the ever possible neglect to manoeuvre the sluice. At the last agricultural exhibition at Tulle was shown a recently devised and very simple system, the great advantage of which is the entire absence of any mechanism whatever subject to get out of order. It is a siphon, but it has been so arranged by Delavallade that the problem is entirely solved despite all the difficulties enumerated. Fig. 278 gives a general view of the apparatus and the manner in which it is arranged In the sluice hole of a reservoir. Thus placed, and supported by 2 wooden posts, one has no longer to pay any attention to it. Whatever be the irregularities in the discharge of the source, the siphon will never act as a waste pipe, and will prime jtself as soon as the desired level of water is reached in the reservoir.