xix.] EXPERIMENT. 431 



the interference of wind, which deflects more or less rain 

 from all the gauges which are exposed to it. 



The great magnetic power of iron renders it a source of 

 disturbance in magnetic experiments. In building a mag- 

 netic observatory great care must therefore be taken that 

 no iron is employed in the construction, and that no 

 masses of iron are near at hand. In some cases magnetic 

 observations have been seriously disturbed by the existence 

 of masses of iron ore in the neighbourhood. In Faraday's 

 experiments upon feebly magnetic or diamagnetic substances 

 he took the greatest precautions against the presence of 

 disturbing substances in the copper wire, wax, paper, and 

 other articles used in suspending the test objects. It was 

 his custom to try the effect of the magnet upon the appa- 

 ratus in the absence of the object of experiment, and with- 

 out this preliminary trial no confidence could be placed in 

 the results. 1 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. 2 It is worthy of notice that in the very 

 infancy of the science of magnetism, the acute experimen- 

 talist Gilbert correctly accounted for the opinion existing 

 in his day that magnets would attract silver, by pointing 

 out that the silver contained iron. 



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 an experiment is of really high importance, 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 weak point. Especially when our results are more 

 regular 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 always in- 

 dicates the movement of the axes of its own elliptic path 

 instead of the rotation of the globe. 



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

 * Lectures on Heat, p. 21. 



