VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 145 



made either to attract or repel, by moving it up or down only half an inch, so 

 as to appear to change its attraction or repulsion at command. This polarity of 

 position may be very sensibly perceived, by presenting small nails, or smaller bits 

 of wire, above or below the needle, or with the remote end inclining towards 

 the north or south ; which plainly demonstrates the existence of a magnetic at- 

 mosphere over the earth, where the magnetic fluid being rarefied at one pole, 

 and condensed at the other, occasions the polar direction of the needle, of so 

 much use in navigation. 



Exper. 16. On reading Mr. Cavallo's experiments on the increased attraction 

 of iron by effervescence, Dr. Darwin was led to inquire, whether inflammable 

 air be magnetic. Mr. B. therefore, at his request, caused inflammable air to 

 issue through a paper tube held near the north and south pole of the needle alter- 

 nately; the air was also received in a bladder, and applied; but without producing 

 any sensible effect on the needle. In the 76th vol. of the Philos. Trans., Mr. 

 Cavallo has endeavoured to prove, that brass " does not owe its magnetism to 

 iron, but to some particular configuration of its component particles, occasioned 

 by the usual method of hardening it, which is by hammering." Some brass, he 

 observes, will not acquire " any sensible magnetism by hammering." And in 

 other pieces, which have often passed from the workshop to the furnace, and 

 from the latter to the former, there is contained iron, which renders them mag- 

 netic. Now, since some brass is evidently magnetic because it contains iron, it 

 appears likely that brass, whose magnetism is made sensible by hammering, con- 

 tains a smaller quantity of iron, and that hammering renders it sensible, by 

 giving it some degree of polarity. Therefore no brass can acquire this property 

 which contains no iron. This will appear more evident by the following expe- 

 riments. 



Exper. 17. Mr. B. placed an iron nail, about 2 inches long, in the fire, 

 where it became red-hot, and cooled, as the fire went out, in a position east and 

 west with respect to the magnetic meridian ; by which it became very soft, and 

 when presented towards the needle, it attracted or repelled according to its posi- 

 tion, having no fixed polarity. The nail was then placed on an anvil, with the 

 point directed to the south of the magnetic meridian; and after hammering in 

 this position till it was considerably hardened, its point possessed a fixed south 

 polarity; the other end, being thicker, did not seem to be altered. Another 

 nail was hammered with its point towards the north, which gave it a fixed north 

 polarity. The polarity of these hammered nails might be instantly changed, by 

 bending the point, while held in a contrary position to that in which they were 

 hammered. Several oblong pieces of magnetic brass were hammered in the same 

 manner, and thus made to possess a north or south polarity, according to their 

 position while hammered. Hence it appears, that the general effect of hammer- 



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