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XVI. Magnetical Observations made in the West Indies, on the North Coast of Brazil 

 and North America, in the Years 1834, 1835, 1836, and 1837^ by Captain Sir 

 EvERARD Home, Bart. R.N. F.R.S. Reduced by the Rev. George Fisher, 

 M.A. F.R.S. 



Received November 21, — Read November 23, 1837. 



XT was the object in the experiments recorded in this paper, to determine the rela- 

 tive magnetic forces soliciting both the dipping, and horizontal needles, by observing 

 the times of their completing a given number of vibrations at the various places 

 visited during a period of three years, on the North American and West India Sta- 

 tion, in Her Majesty's Ship Racehorse.^ 



The dipping instrument used was one of modern construction by Dollond. Each 

 observation for the dip consisted of an equal number of readings of the positions of 

 the needle, with the face of the instrument east and west, before and after the inver- 

 sion of the poles, and a mean of all the readings taken for the true dip. The instru- 

 ment had two needles fitted to it, one of which being used solely for the purpose of 

 observing its vibrations, its magnetism was therefore never interfered with, and this 

 needle in this paper is distinguished by the letter B. The other needle was kept for 

 the purpose of determining the dip, and the results obtained with it are given in 

 Table I. 



For the purpose of determining the horizontal forces, besides the needle B, four 

 other needles, Nos. 1,2, 3, and 4, were used, the first two, four and a half, the others, 

 three inches in length, and of a cylindrical form. When used they were suspended 

 by fibres of raw silk in an apparatus constructed for the purpose. They had fixed 

 centres of suspension, with sliding weights for their horizontal adjustments. The 

 vibrations were confined to the same limits of arc, so as to render the experiments as 

 comparative as possible, and to avoid any correction for circular arc. The state of 

 the thermometer was also registered, though at present it is impossible to assign 

 with any degree of accuracy the correction due to the results arising from the dif- 

 ferences of temperatures at which the experiments were made. 



The needles Nos. 1, 2, 3, and 4 appear, by comparing the experiments made near 

 London previous to the voyage, with similar ones at the same place at the expiration 

 of it, to have lost during the interval some portion of their magnetism ; the same 

 circumstance is also observable from the experiments made at Bermuda and other 

 places. To compensate for this change in the intensities of the needles, the experi- 

 ments contained within the horizontal lines in the Tables in which they are recorded 



