shilling] 



PHYSICAL NATURE-STUDY 



293 



reference to liquid air proves that it is a material substance with 

 weight and so i(r^h.. 



The weight and pressure of 

 air form the subject for dis- 

 cussion which may lead* as far 

 into the realm of meteorology 

 as the teacher's judgment shall 

 dictate. A barometer should 

 certainly be used here and the 

 effects of change of weather 

 and of differences in elevation 

 observed. A home-made one 

 consisting of a glass tube filled 

 with mercury and inverted in 

 a small dish serves the purpose 

 admirably. Fig. i. 



In the study of machinery 

 the objective point should be 

 to make plain the advantage 

 to be gained either in speed or 

 in power by the use of the 

 simple machines shown in 

 Fig. 2. Methods of demon- 

 strating these machines are suggested in Fig. 3. 



Fig. 2. 



