C 27s ] 
XVIII, On the secondary d^ections produced in a magnetized 
needle by an iron shell, in consequence of an unequal distribu¬ 
tion of magnetism in its two branches. First noticed by Captain 
J. P. Wilson, of the Honourable East India Company’s Ship 
Hythe. By Peter Barlow, Esq. F. R. S. Mem. Imp. Sc. 
Petrop. 
Read May 17, 1827. 
Captain Wilson, in a very early stage of my magnetic 
experiments, took considerable interest, not only in their 
application to the purposes of navigation, but in the funda¬ 
mental laws on which that application was founded; and in 
pursuit of his own particular views he undertook, in his last 
voyage, to decide, by actual experiment, some of the points 
which were not in the beginning universally admitted; 
amongst which one of the most important, was that relating 
to the change of position of the ideal magnetic sphere which 
I had imagined for the purpose of magnetical reference, and 
for the convenience of reducing the laws of action to their 
most simple and general form. According to the results 
which I had obtained, it was presumed, but not confidently 
asserted, (see page 65, 1st edition “ Essay on Magnetic 
Attractions") that this sphere, in different parts of the world, 
would take up different positions with regard to the horizon, 
following in all cases the changes of position of the dipping 
needle. Captain Wilson proposed to repeat these experi¬ 
ments at different places, where he might have an oppor¬ 
tunity, on his outward and homeward voyages; viz. at St, 
