To illustrate the advantage of having a number of comparatively small valves instead of one large one, suppose a pump to be fitted with four 3-1/2-inch delivery valves at each end, the valves covering ports 2-1/2 inches in diameter. The area of each port is 4·9 square inches. In order to provide an equal area between the valve and the seat the valve must rise a distance equal to one-fourth the diameter of the port. The combined area of the four ports is 19·6 square inches, which corresponds to the area of a circular opening 5 inches in diameter, one-fourth of which is 1-1/4 inches. It will be understood that the smaller valves can seat much more quickly and with less jar than the larger one, hence a larger number of small valves is not only better because of the great reduction in slippage, but they are also more economical, being subjected to less wear and tear. _The lift of valves_ for moderate or low speed pumps is seen in Fig. 190 and those for higher speeds in Fig. 191. These engravings clearly show the relative position of the suction and discharge valves during the movements of the piston. [Illustration: FIG. 190.] [Illustration: FIG. 191.] _Pump slip_ or slippage is a term used to denote the difference between the calculated and the actual discharge of a pump, and is generally expressed as a percentage of the calculated discharge. Thus, when the slippage is given as 15 per cent. it indicates that the loss due to slip amounts to 15 per cent. of the calculated discharge. Slippage is due to two causes, the time required for the suction and discharge valves to seat. When pumps run very fast the piston speed is so high that the water cannot enter the pump fast enough to completely fill the cylinder and consequently a partial cylinder full of water is delivered at each stroke. _High speeds also increase slippage_, due to the _seating of the valves_. Fig. 191 represents a sectional view of the water end of a pump, showing the position of the valves during a quick reversal in the direction of the arrows, which illustrates the position of the valves corresponding to high speed. The valves in a pump, like almost every other detail in the operation of machinery, do not act instantaneously, but require time to reach the seats.