144 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792, 



2 bits of straw joined together in the form of the letter t inverted, it would 

 turn towards any person who approached it at the distance of 3 feet, and would 

 move so rapidly towards wires only heated by the hand, as very much to resemble 

 magnetic attraction. 



Eoi'per. 8. Having found that a spider's thread, only 2-^ inches long, when 

 twisted by above 1 8,000 revolutions, would not cause a sensible deviation of the 

 magnetic needle, owing to its very great tenuity, or to its glutinous quality pre- 

 venting its having any tendency to untwist; and that light substances suspended 

 by it, and inclosed in a glass, were capable of being turned about by so small a 

 degree of heat as that occasioned by a person sitting at the distance of 3 feet 

 from the instrument; or by wires, or other substances, only warmed by holding 

 in the hand; and that when the instrument was placed in a cool room, a slight 

 touch with the end of a finger would cause the wing of the dragon fly, or even 

 a bit of straw, to point exactly at the side of the glass which had been touched; 

 there could remain no doubt of the freedom with which a magnetic needle would 

 move when thus suspended: yet another experiment more directly proves its 

 freedom of motion to be greater than that of former methods. 



Exper. Q. Six rings of horse-hair, made exactly according to Mr. Cavallo's 

 direction, were suspended in a cylindrical glass jar; to the lowest of these rings 

 a spider's thread, 3 inches long, was attached. This thread was fastened to a 

 gold wire twisted round the middle of a small sewing needle. The jar was placed 

 with its mouth downwards, and over the edge of a table, the needle hanging a 

 little lower. After the needle and rings of horse-hair were perfectly at rest, the 

 point of the needle was struck with the end of a finger, which caused it to turn 

 round very swiftly, yet this twisting did not move the rings of horse-hair. A 

 harpsichord wire, 21 inches long, was suspended by 10 spider's threads, to the 

 lowest ring of the horse-hair chain; this was also frequently turned round with- 

 out moving the rings. A wire of this length was afterwards suspended by spider's 

 threads in a proper frame, and with an ivory scale of degrees, with an intention 

 to observe the daily variation; but it was too much influenced by heat, which 

 Mr. B. has not yet been able to obviate. 



After some other experiments of a similar nature, Mr. B. proceeds to some 

 uses of this suspension of the needle, as follows. 



Exper. 13. The first use made of the needle, suspended as above, was to try 

 the polarity of several iron utensils; and, as might be expected, they attracted 

 or repelled the north end of the needle, according to their position with respect 

 to the magnetic atmosphere of the earth. A bar of soft iron, half an inch 

 square, and 9 inches long, moved the needle very sensibly at the distance of 

 about 3 feet; longer bars moved it at a much greater distance; and if a bar was 

 held horizontally, near the end of the needle, and at right angles, it might be 



