skilling] 



PH YSICA L NA T URE-ST UD Y 



293 



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

 weight and so fo~+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 

 obser\^ed. 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 :mach.inery 

 the objective point should be 

 to inake 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. I 



