Double 4 (pi. 66) exhibits a double turbine, or the combination of a turbine moved from the exterior and one moved from the interior. The water, whose influx into the casing C, C' is at C", passes into the directrices D, D, from the exterior, but reaches the lower turbine-wheel from the interior, by which arrangement it is possible to place the cylindrical gate/ (shown partly cut away) on the exterior of the directrix apparatus. The rack-and-pinion rods A, A" serve to raise and lower the cylindrical gate.
Henschel' s all the previously-described forms of construction may be deduced from Segner's reaction-wheel, Euler's wheel forms the original type of a system in which the paddles arc arranged as shown in vertical section in Figure 6. The water does not here run horizontally, as in Figure 5, but vertically downward through the guides and turbine-wheel. Euler also was the first to introduce the guide tubes or tube directrices, and, by a suitable construction of the wheel, to give to the water the above-mentioned vertical direction. The water, however, passes out nearly horizontally after moving over the vertical course. Euler's wheel was imperfectly constructed, because it consisted, like Segner's turbine, of separate curved tubes instead of a number of closely-placed channels, by which arrangement the wheel would have the form shown in Figure 7, which consists of two rims with curved paddles placed between them. The Unproved form of construction exhibited in the latter Figure is the only one used at the present time, and, according to Rahlmann's historical researches, was first introduced in 1837 by Henschel of Cassel, and not in 1840 by Fontaine, as claimed by the French. Fontaine can only lay claim to the invention of the sluice (A/. 66, Jig. 6), which admits of shutting off each separate tube or directrix.
The turbines represented on ('late 66 (jiffs. 7, 8) and Plate 67 (figs. 13, 14) are built according to Henschel's system, incorrectly called Vontaine's, and still more incorrectly Callon's system, which was invented still later. Figure 8 (A/. 66) is the most usual construction. The turbine casing with its supports (13) rests upon the foundation A. In the bottom of the water chamber or timber wheel-box (shown in cross-section) is built, with a water-tight joint, the turbine casing enclosing the cast-iron guide or directrix apparatus (F). The latter consists of three circles or rings of directrices lying one within the other, but cast in one piece, each of which is similar in form to the turbine-wheel in Figure 7. Close beneath the
directrix apparatus are three turbine-wheels, also cast in one piece, whose paddles, however, are curved in a direction opposite to those of the guide, of which arrangement Figure 6 is an example. The turbine-wheel is, of course, connected by a series of anus with the shaft, which passes, water tight (at R), through the upper plate of the directrix apparatus. The shaft, however, consists of a tube which is suspended from the top by means of a bearing, and extends from the foundation to the bevel gear wheel A", which drives the bevel-wheel L. The internal ring of the triple-guide is provided with clack-valves, and the central ring with a cylindrical gate (A'), which can be moved by the rack and gear mechanism 0, P, S, Q. The external ring always remains open, while the two interior rings are opened as required, either for greater effect or on account of less supply of water. The direction of the influx of the water to the wheel box, and that of its efflux through the lower channel, arc indicated by the arrows.
Figure 14 (o/. 67) represents a form of construction similar to the pre ceding, with the exception of an iron water-chamber for the wheel. In front is seen 'the throttle or admission-gate A in the influx of the wheel box, and by the removal of one of its sides (P) a view of the directrix apparatus is obtained. The wheel-casing, as in Figure 3 (o/. 65), is closed by a cover which is provided with a stuffing-box for packing the shaft passing through it, and this arrangement, for the same reason given in describing the latter figure, is a high-pressure turbine. In Figure 13 (pl. 67) there is no directrix apparatus, the turbine-wheel (pl. 66, jig. 7) being placed immediately under the helical influx casing, from which the rather meaningless term " helix " turbine has been applied to this con struction.
Henschel's construction gave rise also to the invention of turbines with long helical paddles or with wheels consisting of a screw (o/. 67, jig. 12). A prominent feature of Henschel's invention, however, is the draft tube, an extension of the casing or discharge-tube into the tail water, which has been claimed by French writers to be the invention of one of their countrymen, and to which the term " Jonval " turbine has been applied, as also to an entire system of turbine-wheels.