228 Introduction to the Study of Science 



in pressure is conspicuous. What is the cause of the difference 

 in the rate of flow? 



Friction in pipes. Water is retarded in its flow by the 

 lengths, varying diameters, inner surfaces, and bends or angles 

 of the pipes. These conditions, with possible air resistance in 

 pipes, reduce the original pressure. All these factors are in- 

 cluded in the word friction. 



The amount of friction due to the inner surfaces of pipes 

 depends upon the materials of which the conduits are made. It 

 varies with wood, iron, steel, cement, and other materials. 

 Wooden pipes, such as are generally used on the Pacific coast, 

 give, when new, less resistance than iron or steel pipes, and 

 become smoother and offer still less resistance after being used 

 a few years. Iron and steel pipes, on the contrary, resist in- 

 creasingly with use. Rivets in steel pipes cause additional fric- 

 tion. These facts indicate some of the obstructions to the flow 

 of water, which are due to different kinds and ages of materials. 



The loss of pressure due to length of pipe is considerable. A 

 pipe 10,000 feet long and 1 foot in diameter, with a fall or head of 

 100 feet, delivers about 4.25 cubic feet of water a second. At 

 this point it would throw vertically a jet of the same volume 

 only a little higher than the upper surface of the pipe. At the 

 reservoir a jet of water of the same volume would, if directed 

 vertically, reach very nearly the 100 foot level. The pressure 

 10,000 feet away is thus perceived to be very much less than 

 that at the reservoir. A working rule is that the loss of pres- 

 sure through friction is directly proportional to the length of 

 the pipe. 



In laying long pipes it is customary to use pipe of large 

 diameter, because the larger the diameter of the pipe, the less 

 will be the friction, or the greater the flow. There is loss of 

 pressure due to the entrance of water into the pipe at the res- 

 ervoir. In determining, therefore, the rate of discharge of water 

 at any place in a system, it is necessary to take into consideration 

 all these factors which cause friction or retard the flow. 



