196 Things not generally Known. 



blows they receive, have acquired some magnetic virtue, which 

 on trial he found to be the case ; and therefore he employed 

 them to impregnate his bars, instead of having recourse to the 

 natural loadstone. Upon the reading of the above paper, Can- 

 ton exhibited to the Royal Society his experiments, for which 

 the Copley Medal was awarded to him in 1751. 



Canton had, as early as 1747, turned his attention, with 

 complete success, to the production of powerful artificial mag- 

 nets, principally in consequence of the expense of procuring 

 those made by Dr. Gowan Knight, who kept his process secret. 

 Canton for several years abstained from communicating his 

 method even to his most intimate friends, lest it might be 

 injurious to Dr. Knight, who procured considerable pecuniary 

 advantages by touching needles for the mariner's compass. 



At length Dr. Knight's method of making artificial magnets 

 was communicated to the world by Mr. Wilson, in a paper 

 published in the 69th volume of the Philosophical Transactions. 

 He provided himself with a large quantity of clean iron-filings, 

 which he put into a capacious tub about half full of clear 

 water ; he then agitated the tub to and fro for several hours, 

 until the filings were reduced by attrition to an almost im- 

 palpable powder. This powder was then dried, and formed 

 into paste by admixture with linseed-oil. The paste was then 

 moulded into convenient shapes, which were exposed to a mo- 

 derate heat until they had attained a sunicient degree of hard- 

 ness. 



After allowing them to remain for some time in this state, Dr, 

 Knight gave them their magnetic virtue in any direction he pleased, 

 by placing them between* the extreme ends of his large magazine of 

 artificial magnets for a second or more, as he saw occasion. By this 

 method the virtue they acquired was such, that when any one of these 

 pieces was held between two of his best ten-guinea bars, with its poles 

 purposely inverted, it immediately of itself turned about to recover its 

 natural direction, which the force of those very powerful bars was not 

 sufficient to counteract. 



Dr. Knight's powerful battery of magnets above mentioned 

 is in the possession of the Royal Society, having been presented 

 by Dr. John FothergiU in 1776. 



POWER OF THE SUN'S BAYS IN INCREASING THE STRENGTH 

 OF MAGNETS. 



Professor Barlocci found that an armed natural loadstone, 

 which would carry 1 Roman pounds, had its power nearly 

 doubled by twenty-four hours' exposure to the strong light of 

 the sun. M. Zantedeschi found that an artificial horse-shoe 

 loadstone, which carried 13 oz., carried 3J more by three days' 

 exposure, and at last arrived to 31 oz. by continuing it in the 

 sun's light. He found that while the strength increased in 



