HYDRAULICS. 



151 



\Vhat wiU be 

 the difference 

 in the flow of 

 B liquid when 

 the vessel is 

 kept full and 

 when it is al- 

 lowed to emp- 

 ty itself? 



What is the 

 principle and 



construction 

 of the water- 

 clock? 



Fig. 143 



A 



As the velocity with which a stream issues depends upon the height of the 

 cohimn of fluid, it follows that when a liquid flows from a reservoir which is 

 not replenished, but the level of wliich constantly descends, its velocity will 

 be uniformly retarded. The following prmciple has been established : — 



339. If a vessel be filled with, a liquid and 

 allowed to discharge itself, the quantity issu- 

 ing from an orifice in a given time, will be 

 just one hiilf what would be discharged from 

 the same orifice in the same time, if the vessd 

 was kept constantly full. 



340. Before the invention of clocks and 

 watches, the flow of water through small ori- 

 fice's was applied by the ancients for the meas- 

 urement of time, and an arrangement for this 



purpose was called a Cl'qysydra, or water-clock. One form of 

 this instrument consisted of a cylindrical vessel filled with 

 water, and furnished with an orifice which would discharge the 

 whole in twelve hours. If the whole depth through which the 

 water in the vessel would sink in this time be divided into 

 144 parts, it will sink through 23 in the first hour, 21 in the 

 second, 19 in the third, and so on, according to a series of odd 

 numbers : this diminishing rate depending on the constantly 

 decreasing height and pressure of the column above the point 

 of discharge. The spaces indicated upon a scale attached to 

 the side of the vessel and compared with the position of the 

 descending column, marks the time. Fig. 143 represents the 

 form of the water clock. 



3-il. The force of currents, whe- 

 ther in pipes, canals, or rivers, is 

 more or less resisted, and their velocity re- 

 tarded, by the friction which takes place be- 

 tween those surfaces of the liquid and the solid which are 

 in contact. 



This explains a fict which may be observed in all rivers: 

 that the velocity of a stream is always greater at the center 

 than near the bank, and the velocity at the surface is greater 

 than the velocity at the bottom. 



342. If a given quantity of liquid must pass 

 through pipes or channels of unequal section 

 in the same time, its velocity will increase as 

 the transverse section diminishes, and dimin- 

 ish as the area of the section increases. 



How is the ve- 

 locity of water 

 in pipes and 

 rivers retard- 

 ed ? 



At what part 

 of a Btrenni is 

 the velocity 

 greatest? 



In a channel of 

 iinoqual sec- 

 tion, how will 

 the velocity of 

 a current be 

 affected ? 



