524 Royal Sociely. 



ployed for determining the direction and the dip of the needle, but 

 more especially the latter. Captain Back, immediately on his return, 

 placed all his magnetical observiitions at the disposal of Mr. Christie, 

 who having since completed their reduction, gives, in the present 

 paper, the results of his labours. 



The first part of the paper relates to the observations of the Dip of 

 the magnetic needle. With a view to economize as much as possible 

 the time consumed in making each observation, the process of invert- 

 ing the poles of the needle, which is usually resorted to in each in- 

 stance, was here dispensed with. But in order that the dip may be 

 determined independently of this operation, it is necessary not only 

 that the position of the centre of gravity of the needle employed should 

 be ascertained, but that it should be permanent. In giving an account 

 of the observations made to verify this condition, the author com- 

 mences with those at Fort Reliance, which was the first winter station 

 of the expedition ; and where the dip was determined by observations 

 of the needle, both with direct and also with inverted poles. The author 

 then enters upon an investigation of formulae for the determination of 

 the dip by means of a needle, in which the value of a certain angle, 

 denoted by the symbol y, determining the position of the centre of 

 gravity, has been ascertained ; and, conversely, for the determination 

 of the value of the same angle, or, which is equivalent to it, the posi- 

 tion of the centre of gravity of the needle, when the dip at the place 

 of observation is given. He next inquires whether any tests can be 

 applied to the observations under discussion, which may indicate the 

 extent of the errors by which the results deduced from them nray be 

 affected ; and he employs for this purpose the values of the terrestrial 

 magnetic intensity furnished by certain equations obtained in the pre- 

 ceding investigation ; making the proper allowances, first, for the 

 needles used being ill adapted to this method of determining the re- 

 lative intensities; secondly, for errors of observation in determining 

 the times of vibration of the needle ; and thirdly, for disturbing causes 

 which might afi^'ect the observations. Considerable dift'erences were 

 found to exist in the results obtained by the two methods, at New 

 York, Montreal, Fort Alexander, Montreal Island, and Fort Ogle ; 

 difierences which can be accounted for only by errors in the assumed 

 magnitude of the angle y, and which, consequently, indicate the want 

 of permanence in that angle. It was necessary, therefore, to inquire 

 what changes in the angle y will account for these discrepancies, and 

 how far the value of the dip, thus obtained, may be aflected by them. 

 Formulae are then deduced by which these changes may be deter-' 

 mined. 



From a comparison of the observed and computed values of the 

 angles involved in these investigations, the author infers that, the dif- 

 ferences between those of one of these angles are, with a few excep- 

 tions, contained within the limits of the errors incident to dip obser- 

 vations : but with respect to the other angle, they in general exceed 

 those limits. Upon the whole, he concludes that the discrepancies 

 which appear between the values of the terrestrial intensity, as de- 

 duced from the times of vibration of the needle, and from the observed 



