ADVANTAGES OF VARIETY OF EXPERIMENTS. 383 



quite adequate to obtain the truth ; but it may sometimes 

 happen because the materials and means which are used 

 to carry it out practically are not adapted to that purpose ; 

 and although these experiments cannot contaminate the 

 purity of the theoretical speculations, they are nevertheless 

 unfitted to second them, on account of the materials 

 employed.' ! 



In an investigation, it is usual to exclude the largest 

 errors first, because it is not of much use to exclude small 

 ones if large ones remain ; and it is highly important to 

 reduce the mode of experimenting to its purest and 

 simplest form as quickly as possible, because until that 

 point is attained the results of the experiments are more 

 or less untrustworthy, and, no matter what time or money 

 they may have cost, have to be wholly or partially 

 discarded. 2 The mode of experimenting cannot, how- 

 ever, be reduced to its purest form until such a variety of 

 experiments have been made as to disclose the existence 

 of all the interfering circumstances. Some interferirlg 

 circumstances are very treacherous sources of error, 

 because they are unsuspected ; such are usually dis- 

 closed by a want of uniformity in the results, even when 

 the conditions are apparently similar ; and this want 

 of uniformity is produced by the previously unsuspected 

 cause or causes operating according to a different law of 

 variation to that of the cause of the pure phenomenon. 

 As an example of this kind ; I was investigating the 



1 Magalotte. 



2 I believe that Mr. Bailey, in his experiments made to ascertain 

 the mean density of the earth (commonly known as ' weighing the 

 earth '), after several years of labour and great expense, discovered 

 an error which vitiated all the results ; and he therefore discarded 

 the whole of them, and repeated the entire series of experiments over 

 again, with the source of error excluded from them. 



