Turbines may also be divided into _reaction turbines_, or those actuated substantially by the water passing through them (their buckets moving in a direction opposite to that of the flow); _impulse turbines_ or those principally driven by impact against their blades or buckets (the buckets moving with the flow); and _combined reaction and impulse wheels_ which include the best modern types of turbines. In reaction turbines the wheel passages are designed to be always full and therefore the water under pressure; in the impulse turbine the passages are not usually full. [Illustration: FIG. 119.] Turbines in which the water flows in a direction parallel to the axis are called _parallel flow turbines_—or _journal turbines_. The _turbine-dynamometer_ is a device used for measurings or testing the power delivered by turbines (whence its name). _Fourneyron’s Turbine._ This is, in its latest form, when properly constructed, the nearest perfect of the horizontal water-wheels. It revolves either in the air or under water, and may be either high or low pressure. For the low-pressure wheel, the water enters the flume from the open reservoir, with free surface, as in Fig. 119. For high pressure, the reservoir is boxed up and the water brought in at the side through a pipe, as shown in Fig. 124, page 136. The first is for low and the second for high falls. NOTE.—The early history of the turbine is one of considerable interest _especially in view of the development of the steam, from the water turbine_. “M. Fourneyron, who began his experiments in 1823, erected his first turbine in 1827, at Pont sur l’Ognon, in France. The result far exceeded his expectations, but he had much prejudice to contend with, and it was not until 1834 that he constructed another, in Franche Comté at the iron-works of M. Caron, to blow a furnace. It was of 7 or 8 horse-power, and worked at times with a fall of only 9 inches. Its performance was so satisfactory that the same proprietor had afterwards another of 50 horse-power erected, to replace 2 water-wheels, which together, were equal to 30 horse-power. The fall of water was 4 feet 3 inches, and the useful effect, varied with the head and the immersion of the turbine, 65 to 80 per cent. Several others were now erected: 2 for falls of seven feet; 1 at Inval, near Gisors, for a fall of 6 feet 6 inches, the power being nearly 40-horse, on the river Epté, expending 35 cubic feet of water per second, the useful effect being 71 per cent. of the force employed.” [Illustration: FIG. 120.] _The Leffel-Samson turbine_ wheel is shown in the engraving, Fig. 120, page 129, where _N.N._ represents the bottom casting of the case flanged to support the wheel by resting upon the bottom of the penstock.