494 
COMMANDER BELCHER’S MAGNETICAL EXPERIMENTS 
between the same limits of arc, since the value of the result depends upon the 
permanency of the magnetism in the needles, in order that the experiments 
made with them may be strictly comparative. 
The observations were frequently repeated in different places, at some 
distance from each other, at each station, and a mean of these taken as one 
result, as at Goree and Rio Nunez. The observations at Bathurst were made 
in the Government-house, and also at some distance from it, but exhibit no 
material difference. 
By thus varying the place of observation at each station, a mean result is 
obtained, which is most probably more free from errors, particularly those 
arising from irregularities caused by the vicinity of iron ores and other 
peculiarities of the soil ; an instance of which took place at the Isles de Los. 
Captain Belcher observes, that “ these islands being of volcanic origin, the 
sands even contain iron sufficient to influence the needle, and the rocks in 
some positions so forcibly, as to cause one of the needles suspended horizon- 
tally, to cease almost instantaneously after twenty vibrations.” 
The whole detail of Captain Belcher’s experiments is very extensive ; the 
following tables therefore contain only the abstract of them, together with the 
results which I have deduced from them. They do him the greatest credit, 
and evince his indefatigable exertions, as well as excellent judgement. 
Table I. contains the means of a great number of observations obtained by 
observing the times of completing a certain number of vibrations with the re- 
spective needles vibrating between the same limits of arc, viz. 30° and 10°. 
Table II. contains the horizontal forces at the different places, considering 
the horizontal force at Portsmouth equal to unity, and computed from the 
formula <p varies as — , where t is the time of completing n vibrations of the 
needle, when solicited by the force <p. The needles were suspended by a few 
fibres of silk. 
