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deliver to a higher elevation than the 1-inch pipe. If the interior 

 surface of all pipes were so. smooth as to be frictionless there 

 would be no difference, but the proportion of resisting surface to 

 the cross-sectional area of the passing water is so much greater 

 in the 1-inch pipe than in the 2-inch pipe that the velocity is much 

 slower and consequently the pressure much less. 



This relation between- the area and interior surface is known 

 in hydraulics as the "hydraulic radius," and is an important factor 

 in computing discharges. It is also used in computing the flow 

 in open channels when the computation is based on the gradient 

 or slope of the channel. 



To measure the flow in a pipe or other conduit when the water 

 is under pressure, a mechanical device of some sort which regis- 

 ters outside of the pipe the pressure of the water within, must be 

 installed. These mechanical devices are of many types, varying 

 with the size of the pipe, the pressures exerted, the use to which 

 the water is put, etc. All types cause a small loss of pressure 

 or head, but usually this loss is so small as to be considered neg- 

 ligible. 



WATER METERS. 



Meters used for measuring the quantity of water supplied to a 

 house or factory are of the displacement type ; that is, as the 

 water passes through the meter it displaces or moves a piston, a 

 wheel, or a valve, the motion of which is communicated through 

 a train of clock wheels to dials where the quantity that has passed 

 since a certain time is registered. There is no theoretical way 

 of determining whether or not the readings of the dial hands 

 are correct, but each meter must be rated by measuring the dis- 

 charge in a tank. Several meters may be placed on the same 

 pipe line in this operation, the same discharge then passing 

 through each of them. When impure water passes through a 

 mieter for any length of time deposits are liable to impair the 

 accuracy of its readings, and hence it should be rerated at in- 

 tervals. 



The piston type is one in which the motion of the water causes 

 two pistons to move in opposite directions, the water leaving and 

 entering the cylinder by ports which are opened and closed by 

 slide valves somewhat similar to those used in the steam engine. 

 The rotary meter has a wheel enclosed in a case so that it is caused 

 to revolve as the water passes through. The screw meter has an 

 encased helical surface that revolves on its axis as the water 

 enters at one end and passes out at the other. The disk meter 

 has a wabbling disk so arranged that its motion is communicated 

 to a pin which moves in a circle. In all these, and in many other 

 forms, it is intended that the motion given to the pointers on the 

 dials shall be proportional to the volume of water passing through 

 the meter. The dials may be arranged to read either cubic feet 

 or gallons, as may be required by the consumers. These meters 



