50 



SCIENCE OF COMMON THINGS. 



Velocity of rivers. 



How we make an aqueduct. 



595 Why is the surface of a fluid at rest always kvel ? 



Because the particles are equally attracted towards 

 the earth ly gravity, and are all equally and perfectly 

 movable among themselves. 



596 How slight a declivity is sufficient to give a running motion to 

 water ? 



Three inches to a mile in a smooth, straight channel, 

 gives a velocity of about three miles per hour. The 

 river Ganges, at a distance of 1800 miles from its 

 mouth, is only 800 feet above the level of the sea. 



97 On what principle are we enabkd to conduct water under ground 

 through irregular tubes ? 



On the principle that water will always rise to an 

 exact level in different tubes, pipes, or vessels communi- 

 cating with each other. 



If we connect together a 

 series of vessels, no matter 

 how various their shapes and 

 capacities, so that water may 

 rise from the main channel, 

 A B, into them, we shall find 

 upon pouring water into one 

 that it will rise to the same 

 Fig- 14. level in all the vessels. 



The dependence of all arrangements for conveying water in aqueducts 

 under ground upon the principle, that water in closed tubes or vessels 

 rises to a uniform level, is clearly shown in Fig. 15 : a, a, a, represents 

 the water-level of a pond or reservoir upon elevated ground. From this 

 pond a line of pipe is laid, passing over a bridge or viaduct at d, and 



Fig. 15. 



under a river at c. The fountains, at &, 6, show the stream rising to it3 

 level in the pond, a, at two points of very different elevation. 



S98 In what part of a river does the water flow most rapidly f 



In the middle of the stream, at the surface. On the 



