]18 PHILOSOPHICAL TRANSACTIONS. [aNNO 1776. 



The first thing, it seems, that engaged the doctor's attention more particularly 

 to magnetism, was the accident that betel a ship's compass from lightning ; and 

 of which the doctor gave a very circumstantial account to the society. This af- 

 fair led him to consider the structure of the compass more minutely. He pro- 

 cured compass-cards ready armed, as it is called, from different makers both at 

 home and abroad. He found most of the needles strangely erring from due 

 polarity ; some being many points to the west, others as many to the east of the 

 right position. Among thcni all there was only one, which to him seemed con- 

 structed on a rational plan, and was of French make, procured from Marseilles; 

 but even this was not without very evident faults. 



To fix on the proper form of a needle through which the magnetic effluvia 

 could pass with the least interruption ; to give the needle such a degree of hard- 

 ness as to retain the magnetic influx the longest, and with the greatest force, 

 were material objects ; and, it seems, with a view to have such a degree of 

 magnetic power at his command, as to force the magnetic virtue through the 

 most consolidated bars, was his first inducement to try whether he could not 

 collect such a magazine of magnetism, as would be sufiicient for every ])urpose 

 of this kind, and at the same time exhibit some new phenomena in physics yet 

 undiscovered. With this view he planned and executed his machine. 



His first attempt however was much smaller; a few bars were laid in the due 

 course of the magnetic flux, and impregnated by constant attrition. To these 

 other bars were successively added, after they had been impregnated, both by the 

 force he could give them by attrition, and what he could derive from the pre- 

 ceding stock collected in the bars. To these he added still fresh bars, till he had 

 formed the whole mass, resting on wheels and pivots, in such manner, as to be 

 easily manageable for the purpose of impregnating the needles he was employed 

 to see prepared, for the service of government, and others, who had generosity 

 enough to think, that the compass, on which depended the lives of the ship's 

 crew, could not be made too perfect, and that it deserved a reasonable compen- 

 sation. It is to the Doctor's ingenuity and indefatigable attention to tiiis useful 

 instrument, that it has acquired among us a degree of perfection unknown to 

 our predecessors. 



When the machine was completed, he still was adding continually to its power. 

 He impregnated every single bar of which it is composed, by repealed attritions, 

 •and applied it to the remaining bars in their magnetic position. After this ope- 

 ration, he always found its efficacy for a season considerably diminished; for the 

 effluvia of each bar, though increased in virtue, seemed not immediately to have 

 acquired a communication with each other. However, it grew always more 

 powerful after each of these operations; and it is more than probable, if a person 

 could be found, who, with e(|ual patience and skill, would at proper distances 



