_The tympanum and noria_ in all their modifications have been considered as originating in the gutter or _jantu_, and _the swape_; while the machine we are now to examine is evidently derived from the primitive _cord and bucket_. The first improvement of the latter was the introduction of a pulley or sheave over which the cord was directed—the next was the addition of another vessel, so as to have one at each end of the rope, and the last and most important consisted in uniting the ends of the rope, and securing to it a number of vessels at equal distances through the whole of its length—and the _chain of pots_ was the result. (See Fig. 80.) [Illustration: FIG. 80.] The general construction of this machine will appear from an examination of those which are employed to raise water from Joseph’s well at Cairo, represented on page 45. Above the mouth of each shaft a vertical wheel is placed, over which two endless ropes pass and are suspended from it. These are kept parallel to, and at a short distance from each other, by rungs secured to them at regular intervals, so that when thus united, they form an endless ladder of ropes. The rungs are sometimes of wood, but more frequently of cord like the shrouds of a ship, and the whole is of such a length that the lowest part hangs two or three feet below the surface of the water that is to be raised. Between the rungs, earthenware vases (of the design shown at A A) are secured by cords round the neck, and also round a knob formed on the bottom for that purpose. WHEEL AND AXLE. In all the preceding machines the roller is used in a _horizontal_ position; but at some unknown period of past ages, another modification was devised, one by which the power could be applied at any distance from the center. Instead of placing the roller as before, over the well’s mouth, it was removed a short distance from it, and secured in a vertical position, by which it was converted into the wheel or capstan. One or more horizontal bars were attached to it, of a length adapted to the power employed, whether of men or animals; and an alternating rotary movement imparted to it, as in the common wheel or capstan, represented in the figure. It appears that machines of this kind, and worked by _men_ were common in Europe previous to, and at the time he wrote. Sometimes the shaft was placed in the edge of the well, so that the person who moved it walked round the latter, and thus occupied less space. [Illustration: FIG. 81.] SUCCESSIVE INVENTIONS. With the wide acceptance in practical use of the Duplex steam pump, may be dated the beginning of the modern inventive period of pumping machinery; this introduction of the Duplex pump was only one of five successive advances which it were well for the student to memorize: 1. The Cornish, 2. The Rotative, 3. The Direct Acting, 4. The Duplex, and 5. The Compounded Steam Pump.