2262. The first requisite for jets or projected spouts, or threads of water, by atmospheric pressure, is a sufficiently elevated source or reservoir of supply. This being obtained, pipes are to be conducted from it to the situations for the jets. No jets, however constructed, will rise as high as the fountain-head; because the water is impeded by the resistance of the air, the friction against the opening of the pipe or adjutage, and its own gravity. It is not easy to lay down data on this head; if the bore of the adjutage be too small, the rising stream will want sufficient weight and power to divide the air, and so being dashed against it will fall down in vapour or mist. If too large, it will not rise at all. The length of pipe between the reservoir and the jet will also impede its rising in a slight degree by the friction of the water on the pipe. This is estimated by P. J. Francois (Art des Fontaines, 137.) at one foot for every hundred yards from the reservoir. The proportion which this author gives to the adjutages, relatively to the conducting-pipes, is one fourth; and thus, for a jet of 4 lines, or a third of an inch, he requires an adjutage of between 4 and; lines, and a conducting-pipe of 1.5 in. diameter; for a jet of 6 or 7 lines, a conducting-pipe of 2 in., and so on. From these data, the height of the fountain and the diameter of the conducting-pipe being given, the height to which a jet can be forced can be estimated with tolerable accuracy, and the contrary; but where the pipes are already laid, and the power of the head, owing to intervening obstructions, not very accurately known, the method by trial and correction, by means of a leaden nozzle, the orifice of which may be readily increased or diminished, will lead to the exact power under all the circumstances.
