214 METHODS IN TEACHING 



with stout string at both connections. In the same way 

 connect the funnel to the free end of the short tube. Now 

 run a straight wire to the bottom of the tube thus formed, 

 leaving a few ind^Hfcrojecting from the mouth of the 

 funnel. Pour the m^Bpy in very slowly, being careful to 

 dislodge air bubbles by means of the wire. When the wire 

 is finally removed, the mercury should a little more than fill 

 the longer tube. Remove the funnel and its rubber connec- 

 tion. Keeping the short tube in a perpendicular position, 

 elevate the longer one until it is parallel with it. Notice 

 as the longer tube assumes the vertical position, that the 

 mercury falls a few inches in it and that there is a corre- 

 sponding rise in the short tube. How much air pressure 

 in the closed end of the long tube ? Pupils readily see that 

 there is none, for there is no air there and the tube is closed. 

 They also see that at the open end of the short tube air 

 pressure acts normally. With a little thought they will be 

 able to see that the weight of the column of mercury is sup- 

 ported by air pressure acting upon the surface of the mer- 

 cury in the short open tube. Would the same result be 

 reached if the tubes were either larger or smaller? Yes. 

 What is the height of the column of mercury supported? 

 (Measure from the level of the column of mercury in the 

 short tube.) If near the sea level, it should approximate 

 thirty inches. A column of mercury one inch square and 

 thirty inches high weighs fifteen pounds. Could we make 

 a water barometer? How long would the longer tube be? 

 Why can not water be pumped with an ordinary lifting 

 pump more than thirty feet above the water level in the well 

 or soil? 



Work in solution and crystallization may be undertaken 





