On the Investigation of Magnetic Intensity by the Oscillations of 

 the Horizontal Needle. By W. Snow Harris, Esq. F. R. SS. 

 Lond. & Ed. 



(Read 6th January 1834 J 



1. The many irregularities to which the vibrations of the ho- 

 rizontal needle are subject, renders the usual method of investi- 

 gating the terrestrial magnetic intensity, although precise in 

 theory, occasionally uncertain in practice. It has been my en- 

 deavour in the following paper, to consider in some measure the 

 most prominent of the causes of these irregularities, with a view 

 of arriving at certain practical deductions, by means of which 

 they may be either greatly palliated, or otherwise avoided. 



2. Of the different causes of disturbance in the state of oscilla- 

 tion of a freely suspended magnet, the following may be consi- 

 dered as the most important : 



1st, Variations in the conditions of the surrounding air, in 

 which the oscillations are performed. 



2d, Influence of changes in the mechanical conditions, inci- 

 dental to the mode of suspending the bar ; and of other mecha- 

 nical changes liable to occur in it. 



3d, Changes in the disposition and intensity of the magne- 

 tism of the bar, from heat and other causes. 



4th, To these may be added the influence of the sun's rays, 

 and of atmospheric electricity, both of which have been considered 

 by many distinguished philosophers to exert very sensible effects 

 on the magnetic oscillations. 



VOL. XIII. PART I. a 



