171 



Pressure of Air and Liquids. 



The previous lessons on air and water will have brought out 

 questions requiring some knowledge of the pressure thev exert) 

 and the consequent phenomena. It is perhaps better to begin 

 with water. Bodies floating in water maj^ be taken as a starting 

 point. Why do they float? Why do some float with more of 

 the mass above the water than otliers, while some sink? What 

 happens to the same floating bodies in liquids of different densi- 

 ties, e. g., lighter or heavier liquids? 



These questions may be made the guide to a series of experi- 

 mental lessons with grades above the fourth. ' 



The last of the above questions is a good one to begin with. 

 For the lesson there will be necessary: A small wooden rod one- 

 half inch or less in diameter, and about five inches in length; 

 some small nails, and a piece of copper-wire for weighting one end 

 of the rod; two fruit jars, one filled with water, the other with a 

 strong solution of salt; some small blocks of wood; and some 

 corks. The lesson may begin with the question of how the things 

 will float in two liquids, water and salt solution. If the pupils 

 can be led to invent an apparatus to test the difference, so much 

 the better. 



This may be done by first trying the blocks of wood or the 

 corks. It will soon appear that from their awkward shapes and 

 instability in the water, they can not give good results. Then 

 some one will be sure to suggest a method good enough. Most 

 likely the rod of wood loaded at one end will be invented by the 

 class. If not, it may now be brought out, and will be appreciated. 

 It is best not to have the rod loaded ready for the experiment, 

 but reserve for the class the loading the of rod just right to make it 

 float upright in the two liquids, and the marking of the scale on it. 

 With this now ready, have them test the liquids. They find that 



