of all known Substances to the Magnetic Influence. 117 



site poles of 0°.3', whilst the calculated action, at 24J focal 

 lengths, or 61 feet 6 inches, was found to be 0°.£^' ! The 

 influence of terrestrial magnetism was now partially neutralized 

 by the juxtaposition of a small magnetic bar, when a series of 

 five experiments gave a mean of 0°.4y'. Another series of six exr 

 periments, with an alteration in the position of the neutralizing 

 magnet, gave a mean deviation of 0°.5 J'. The reality of the in- 

 fluence, therefore, under these circumstance, through a distance 

 of 61 feet of earth, stones, and bricks, was unquestionable. 



Results strikingly satisfactory were also obtained, on subse- 

 quent trials, in the south of Ireland, with the two feet magnets, 

 (13) through solid masses of hmestone-rock. The following ab- 

 stract shews the surprising coincidence of the " observed"" and 

 *' calculated results.*" 



III. THB PRACTICAL APPLICATION OP THB MAGNETIC INFLUENCE 

 IN ENGINEERING AND MINING OPERATIONS, IN ORDER TO THE 

 DETERMINATION OP THE THICKNESS OP SOLID SUBSTANCES 

 NOT OTHERWISE MEASURABLE. 



The foregoing investigations, defective as they are in attain- 

 able accuracy, are nevertheless amply sufficient for the practical 

 application of the present method of determining tlie thickness 

 of solid substances, not otherwise measurable. 



The cases in which this mode of measuring distances seems 

 to be most particularly suitable, and where, it appears to me, 

 its application might often be of considerable importance, chiefly 

 occur in mining. 



