PREFACE. ix 



his attention should be directed to things that he has 

 seen and examined for himself; and unless he has 

 learned by his own experience, at least in a few cases, 

 what experimental evidence means, he will scarcely 

 ever be able to appreciate rightly the evidence to be 

 obtained by reading. 



Another reason for introducing as much practical 

 experimental work as possible into the elementary 

 teaching of physics which, though less fundamental 

 than those already pointed out, is still of great prac- 

 tical importance is the influence which it exerts upon 

 the mental attitude of the learner. The great secret 

 of effectual teaching in any subject is to excite the 

 pupil's interest, so that, instead of being passively re- 

 ceptive, and regarding it as his teacher's business to 

 make him learn, he may actively exert his mind in 

 order to understand the matter in hand. In the case 

 of physics no method is nearly so efficacious for this 

 purpose as that of letting him make apparatus and try 

 experiments with his own hands. The very slowness 

 of the progress which this method makes unavoidable, 

 and the length of time during which a single pheno- 

 menon and the conditions of its occurrence are neces- 

 sarily kept before the mind, are, from an educational 

 point of view, no slight advantages. 



I need scarcely say that in thus laying stress upon 

 the importance of a personal acquaintance with the 

 concrete facts of physics, I have no intention of under- 

 rating the value of the mathematical discussion by which 



