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XLIX. On a Determination of the Horizontal Component of 

 the Earth's Magnetism at Oxford. By R. H. M. BoSAN- 

 quet *. 



THE present investigations arose from the discovery that 

 tangent galvanometers gave different indications in 

 different parts of my laboratory, so that it was clear that 

 determinations of the horizontal magnetic force H were 

 necessary. For the purpose of testing the system employed, 

 as well as on account of the interest of the data involved, use 

 has been made of a room away from the laboratory, which 

 has been made free from iron for the purpose of magnetic 

 experiments. The data here obtained may therefore be 

 regarded as having their undisturbed geographical values. 



Making the observation that the determination of the 

 values of small corrections from the observations themselves 

 is a very uncertain process, I have endeavoured to determine 

 the coefficients of the small quantities concerned in a direct 

 manner, and thus to lessen the demands on the accuracy of 

 the principal observations. 



I will first consider the process of comparing M (the 

 moment of a magnet) with H by causing the magnet, placed 

 at a distance r in the E.W. line, to deflect a needle. 



Using Maxwell's notation, if D be the mean of the tan- 

 gents of four deflections, and the experiments are made at 

 two distances, then the application of the equation 



2M r 8 tan0 



H "i + 4* 



leads to the solution 



2M_ D 1 ^-D a r| _ D 2 j-D^ a a 

 H - r\-r\ ' A2 -D^-D 2 rfi^ 



There is a misprint in Maxwell's A 2 , the denominator of 

 which is written r\—r\, thus equating A 2 , which is a square 

 quantity, to an expression of the fifth order. This appears 

 to have escaped the revisers of the second edition. 



I regard the question of certainty as follows : — The deter- 



2M 

 ruination of -*f- depends on the quantity r 3 tan 0, and on the 



factor containing A 2 . The determination of A 2 from pairs of 

 observations is of a very uncertain character. In fact, the 

 numerator of the above expression for A 2 consists of the 



* Communicated by the Author, having been read before the Ashmo- 

 lean Society, March 10, 1884. 



