IN POPULAR PHYSICS. 29 



9. If the ivater filtering down through a rock should 

 colled in a crevice an inch square and 25O feet high, open- 

 ing at the bottom into a closed fissure having 2 O square 

 feet of surface, what would be the total pressure tending to 

 burst the rock? 



Neglecting the diameter of the fissure, the pressure is the 

 same on every square inch of the twenty square feet of sur- 

 face. 



j^ 250 x 1,000 oz.x 20x144 = 31 

 144 > 1 1^ 



10. Why can stones in water be moved so much more 

 easily than on land? 



Because the water buoys up nearly one half of their 

 weight. 



11. IVliy is it so difficult to wade in the water where 

 there is any current ? 



Because we have to move not only the weight of our own 

 bodies, but also the water. The kinetic energy of this is pro- 

 portional to the mass displaced, and to the square of its velocity. 



1 2O 12. Wliy is a mill-dam or a canal embankment 

 made small at the top and large at the bottom ? 



Because the pressure of the water increases with the depth. 



13. In digging canals, ought the engineer to take into 

 consideration the curvature of the earth ? 



There is no necessity to do so. A water level is in practice 

 assumed to be horizontal. In geodetic surveys, like that of the 

 coast line of a country, allowance has to be made for the cur- 

 vature of the earth. Station points twenty miles apart, or 

 more, are taken, between which the earth's curvature is readily 

 calculable and perceptible. 



14. Why does the bubble of air in a spirit-level move 

 as the instrument is turned ? 



Because the air is lighter than the alcohol, and rises con- 

 stantly to the highest point. For this reason, also, the tube is 

 curved so as to be convex at the center. 



