24 PEEOAUTIONS. CHAP. m. 



greatest amount that are to be observed as indicating 

 the most satisfactory results, but the constant, 

 steady, and definite motion of the needle ; and with 

 respect to the amount of its deflection, a slight rather 

 than one of great extent is that upon which the most 

 trustworthy and satisfactory results are to be placed. 

 Practice alone, however, can only and would soon 

 give that knowledge which cannot be obtained by 

 mere description, and it is by practice alone that any 

 experiment can be safely undertaken and depended 

 upon. We must bear this most important fact also 

 in mind, that it is not the mere circumstance of 

 getting an effect upon the needle that we are to be 

 contented with, but we have to account for the effects 

 when obtained, and to shew to what class of phe- 

 nomena they may be referred. 



