4 o SCIENCE PRIMERS. [MATERIAL 



wide or narrow, the column of water inside it will be at 

 exactly the same level as the water outside it. Yet, of 

 course, the rigid glass walls of the tube cut off all 

 communication between the column of water inside 

 it and the rest, except at the bottom. 



In a well-ordered town, water is supplied to every 

 house and can be drawn from taps placed in the 

 highest stories. These are fed by pipes which lead 

 from a cistern at the top of the house. This water is 

 brought from a large pipe, or main, in the street, by 

 a smaller house-pipe, which is often made to twist 

 about in various directions before it reaches the 

 cistern at the top of the house into which it delivers 

 the water. If you followed the main, you would find 

 that it took a long course up and down, beneath the 

 pavement of the streets, until at last it reached the 

 waterworks. Here you would find that the main 

 was connected with a reservoir ; and either this reser- 

 voir is at a greater height than any of the cisterns 

 into which the water is delivered, or there is some 

 means of pumping the water from it to that height on 

 its way to the main. Thus the reservoir, the main, 

 and the house-pipe form one immense (J-tube, and 

 the water in the house-pipe tends to rise to the same 

 level as that of the water in the reservoir, and hence 

 flows into the cistern when the supply-pipe is open. 



28. The Transference of Motion by Moving 

 'Water : the Momentum of Moving Water. 



Suppose a wooden vat with a horizontal tap, the 

 sectional area 1 of the tube of which is one square inch, 



1 The sectional area of a tube is the surface occupied by its 

 cavity when it is cut across. It would be represented by the 



