197. =Friction of Liquids in Tubes.=--So easily does water flow along that we should not at first view suppose that it would be delayed much from friction as it passes through pipes or along channels. But the retarding influence is considerable. An inch tube 200 feet long, lying horizontally connected with a reservoir, will discharge water not one quarter as fast as an inch orifice in the side of the reservoir. Sudden turns in a pipe should be avoided, because they occasion so much friction against the sides of the pipe and among the particles of water by disturbing the regularity of the current. In the entrance of the arteries into the brain, in order to prevent the blood from flowing too rapidly into this organ, there are sudden turns in the arteries to retard the blood; and in grazing animals, as there is special danger that the blood will flow too freely to the brain as the head is held down in eating, there is a special provision to prevent this in a net-work of arteries. If the arteries of the brain in such animals were straight tubes they would continually be dying of congestion of the brain or of apoplexy. [Illustration: Fig. 131.] Friction in a small pipe is greater in proportion to its size than in a large pipe. In a pipe an inch in diameter water will not move more than one-fifth as fast as in a tube two inches in diameter. This may be made clear by Fig. 131, in which is represented the area of a small tube inside of the area of a tube of twice its diameter. Suppose the effect of the friction in the large tube to extend in to _a_. In the small one it will extend in as far, that is, to _b_. But _e a_ is about five times as long as _e b_, so that there is full five times as much water clear of friction in the large tube as there is in the smaller one. [Illustration: Fig. 132.]