558 FOURTH REPORT — 1834. 



dependent on temperature, — Professor Lloyd proceeded to 

 give an account of a series of observations which had been 

 commenced in Ireland by Captain Sabine and himself, and of 

 the steps which had been taken to carry into effect the recom- 

 mendation of the Association in that country. 



The first object of the observers was to compare, with accu- 

 racy, the direction and intensity of the magnetic force at Dub- 

 lin and Limerick, the two stations from which it was proposed 

 to set out. This was accomplished with much care, the mag- 

 netic intensity at the two stations having been compared by a 

 repeated interchange of needles ; and in this manner a close 

 approximation was made to the direction of the magnetic lines 

 in Ireland, and thus the most favourable points of observation 

 ascertained. The latter of these two stations has recently 

 been compared with London, by an interchange of needles be- 

 tween Captain Sabine and Commander Ross ; and a similar com- 

 parison of the total intensity in London and Dublin has been 

 made by Professor Lloyd, so that the series is thus connected 

 with observations taken elsewhere. The series itself will, it is 

 hoped, be shortly completed, and a connected view of the re- 

 sults laid before the Association at its next meeting. 



Besides the usual method of determining the terrestrial mag- 

 netic intensity suggested by Hansteen, Professor Lloyd adopted 

 another, in which the dip and the intensity are ascertained by 

 the same observation, and with one instrument. This method 

 consists in observing the direction assumed by an ordinary dip- 

 ping-needle under the combined influence of magnetism and 

 gravity. If two small weights be successively attached to the 

 southern arm of the needle, and if ? and & denote the inchna- 

 tions of the needle in the two cases, these angles will be con- 

 nected with the dip and the force by the equations 



/* cos i^= <p (T sin (8—?) 

 V cos fl = ij) (7 sin (S— Q), 



in which /* and v denote the moments of the added weights, 5 

 the dip, <p the force, and cr a constant depending on the distri- 

 bution of magnetism in the needle. Hence, if the two moments, 

 fjL. and V, be known, and if the angles, ^ and fl, be observed, the 

 two unknown quantities, 8 and <|), will be completely determined. 

 The friction of the axle, however, Avhich is the main source 

 of error in the dipping-needle, will affect these quantities differ- 

 ently in different positions. Professor Lloyd has found from 

 theory that the limit of error in the determination of the di^i 

 arising from this cause will be least when the position of the 



