Both pinions are supported by a journal at each end, the shaft of one being extended to receive a pulley or hand wheel as shown in Fig. 166, hence, one pinion causes the other to revolve. The teeth on the bottom side of the pinions move away from each other and form a partial vacuum which the water fills and is carried around between the teeth of the pinions on opposite sides and is discharged through a central opening in the case at the top, thence through the discharge pipe into the tank or reservoir. In the small sizes of the rotary pump there is no trouble from leakage, until the parts become much worn. [Illustration: FIG. 168.] _Bag Pump._ This is a form of bellows-pump in which the valve disc A, which takes the place of the bucket, is connected with the base of the barrel by an elastic bag distended at intervals by rings. This bag may be made of leather or of double canvas. The upper end of the bag should be firmly tied with a cord in a groove gouged out of the rim of the board at A. Into this board is fixed the fork of the piston rod, and the bag is kept distended by a number of wooden hoops or rings of wire, fixed to it at a few inches distance from one another, and kept at equal distances by three or four cords binding them together and stretching from the top to the bottom of the bag. Now let this trunk be immersed in the water: it is evident that if the bag be stretched from the compressed form which its own weight will give it by drawing up the piston rod, its capacity will be enlarged, the valve A will be shut by its own weight, the air in the bag will be rarefied, and the atmosphere will press the water into the bag. When the rod is thrust down again, the water will come out at the valve A, and fill part of the trunk. A repetition of the operation will have a similar effect; the trunk will be filled, and the water will at last be discharged at the spout. [Illustration: FIG. 169.] _Bellows-pump_ (Page 186). This is an atmospheric pump in which the part of the piston is played by the top leaf of the bellows. A very simple method of describing an invention, from which great good in drainage of waste lands in Europe, was realized. “There was of course a valve covering the interior orifice of the nozzle and opening outwards, to prevent the air from entering when the upper board was raised. This valve is not shown because the art of representing the interior of machines by section, was not then understood, or not practiced. The lower board is fastened to the ground by a platform while the suction pipe dips into the water. A weight is placed on the upper board to assist in expelling the water.” Fig. 169 represents a _Double Lantern Bellows-Pump_ as used in the 16th century. This engraving is plain and requires no description. Fig. 170 shows a diaphragm pump in which a sheet of rubber or its equivalent is used as a substitute for a piston in a cylinder. [Illustration: FIG. 170.]