EXPERIMENT. 41 



rie tic substances he took the greatest precautions against 

 the presence of any disturbing substance in the copper 

 wire, wax, paper, and other articles used in suspending 

 the test objects. It was his invariable custom to try 

 the effect of the magnet upon the apparatus in the absence 

 of the object of experiment, and without this preliminary 

 trial no confidence could be placed in the results b . Tyndall 

 has also employed the same mode for testing the freedom 

 of electro-magnetic coils from iron, and was thus enabled 

 to obtain them devoid of any cause of disturbance . It is 

 well worthy of notice that in the very infancy of the 

 science of magnetism, the acute experimentalist Gilbert 

 correctly accounted for the opinion existing in his day 

 that magnets would attract silver, by pointing out that 

 the silver contained ironA 



Even when we are not aware by previous experience of 

 the probable presence of a special disturbing agent, we 

 ought not to assume the absence of unsuspected inter- 

 ference. If, then, an experiment is of really high im- 

 portance, so that any considerable branch of science rests 

 upon it, we ought to try it again and again, in as varied 

 conditions as possible. We should intentionally disturb 

 the apparatus in various ways, so as if possible to hit by 

 accident upon any peculiar weak points. Especially when 

 our results are more regular and accordant than we have 

 fair grounds for anticipating, ought we to suspect some 

 peculiarity in the apparatus which causes it to measure 

 some other phenomenon than that in question, just as 

 Foucault's pendulum almost invariably indicates the re- 

 volution of the axes of its own elliptic path instead of the 

 revolution of the globe. 



It was in this cautious spirit that Baily acted in his 



b ' Experimental Researches in Electricity,' vol. iii. p. 84, &c. 

 c 'Lectures on Heat/ p. 21. d Gilbert, 'De Magnete.' 



