300 



the curve of intensities becoming, in that case, very nearly a 

 right line passing through the centre of the magnet. 

 Employing then this approximate formula, we have 



The ratio of these quantities is — = f ^, a value indepen- 

 dent of ?n ; and substituting in the expression of h above 

 given, and designating the half lengths of the deflecting and 

 of the suspended magnets by I and /', respectively, 

 h = f (2/2-3 ^'2); 



an expression whose value may be exactly known, indepen- 

 dently of experiment. This value vanishes, when P zz ^l'^, or 



Zzz 1-224./'; 



and in this case, therefore, the quantity sought is given by 

 the simple formula 



Q = D^ tan u. 

 The author concluded his paper with an account of a 

 series of deflection experiments, instituted for the purpose 

 of confirming these results. The magnets employed were 

 cylindrical, their lengths being 3 inches and 3f inches, and 

 their diameter 3-lOths of an inch. The observations were 

 made with every precaution necessary to insure exactness, 

 and at times when the fluctuations in the direction and in- 

 tensity of the magnetic force were very small ; and their re- 

 sults verify the conclusions above obtained, as applied to the 

 case of small macrnets. 



Dr. Apjohn next read the following letter, which he had 

 received from Captain Boileau, superintendent of the Mag- 

 netic Observatory at Simla, in India. 



" Simla, March, 7, 1842. 

 " My dear Sir, 



" I have the pleasure of forwarding to you, through the 



Government of India, a complete set of hygrometric tables, 



