S, is the shaft; G G, the gutters; A, a trough to take away the water. The arrow indicates the direction in which the wheel turns; each gutter, as it revolves scoops up a portion of water and elevates it, till by the inclination to the axle, it flows towards the latter, and is discharged through one end of it. The prominent defect of the tympanum arises from the water being always at the extremity of a radius of the wheel, by which its resistance increases as it ascends to a level with the axis, being raised at the end of levers which virtually lengthen till the water is discharged from them; this has been remedied by making the arms curving as shown in the _Scoop Wheel_ (Fig. 75.) As this revolves in the direction of the arrow the extremities of the partitions dip into the water and scoop it up and as they ascend discharge it into a trough placed under one end of the shaft which is hollowed into as many compartments as there are partitions or scoops. [Illustration: FIG. 75.] Fig. 76 represents a sectional view of _an improved tympanum_; this was invented by De La Faye; the illustration will be readily understood. As shown in Figs. 74 and 75 the wheel is driven by the current of a stream impinging upon what in later times came to be known as boards or floats on the circumference of the wheel. [Illustration: FIG. 76.] Within the enclosure are arranged four scrolls of suitable proportions, dipping the water, at one end, and emptying it out at the center of the wheel as more clearly shown in Figs. 74 and 75. _The Noria or Egyptian Wheel._ The tympanum has been described as an assemblage of gutters, and the _Noria_ may be considered as a number of revolving swapes. It consists of a series of poles united like the arms of a wheel to a horizontal shaft. To the extremity of each, a vessel is attached which fills as it dips into the water, and is discharged into a reservoir or gutter at the upper part of the circle which it describes. Hence, the former raises water only through half a diameter, while this elevates it through a whole one. (Fig. 77.) The Chinese make the noria, in what would seem to have been its primitive form, and with an admirable degree of economy, simplicity, and skill. With the exception of the axle and two posts to support it, the whole is of bamboo, and not a nail used in its construction. Even the vessels, are often joints of the same, being generally about four feet long and two or three inches in diameter. They are attached to the poles by ligatures at such an angle, as to fill nearly when in the water, and to discharge their contents when at, or near the top.