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SCIENTIFIC RESEARCHES,

(TWENTY-SIXTH MEMOIR.)

Moreover, a delicate magnetic needle does not possess a sufficient degree of power tobring into play the minute portions of Magnetism that lie dormant in many bodies.These exiguous sleeping forces can never be roused into a state of activity, and con-sequently can never be discovered, by merely presenting the bodies in which theyreside to the pole of a feeble magnetic needle. To accomplish their discovery, a com-paratively powerful magnetic action is absolutely required; for, when thus assailed,their polarization is more easily enforced, and their detection almost certain. Themagnetic needle, however, may be usefully employed in cases where the suspectedMagnetism of a body is of some easily detected amount; and it may be resorted towith advantage in preliminary trials, under all circumstances, because of the possibilityof the specimen under examination possessing a sufficient amount of Magnetism to bedetected by it, and the more tedious modes of inquiry being thus rendered un-necessary.

16. I have found that a convenient and efficacious mode of examining bodies, themagnetic actions of which are very feeble, and others in which Magnetism has but aquestionable existence, is by means of an apparatus represented by Figs. 13 and 14,Plate XVII. Fig. 13 is that part of the apparatus in which the specimens to beexamined are placed. It consists of a light cylindrical wooden rod, a b, about twelveinches long, and suspended by a few parallel fibres of silk, f f, from the cocoon. Theend, b, is furnished with a light slip of card paper, and two loops of horse-hair, forthe purpose of holding the specimen—say a half-crown, for instance—as representedin the figure, which is counterbalanced at the other end of the lever by a slidingweight. This part of the apparatus is inclosed in a rectangular box, Fig. 14, whoseends, top, and one of its sides are of glass; and a brass tube rises from the middle ofthe top, in which hangs the silken fibres. The head of this tube sustains the fibresand their appendages, and can be turned in any horizontal direction, for the adjust-ment of the lever to a parallelism with the sides of the box.

The glass parts of the box are sustained by a light mahogany frame, with a bottomof the same kind of wood. The ends and side are fixed, but the top, which consistsof two sliding parts, can be removed at pleasure, for the purpose of introducing thehands for the adjustment of the apparatus within, and replaced when the specimenhas been accurately counterpoised—a process which is still further facilitated by theintroduction of a hand at one side of the box, which is opened for that purpose, andafterwards closed by a sliding mahogany door.

When the agitations of the lever have subsided, the sliding door is partially openedfor the introduction of the poles of a powerful horse-shoe magnet, which is made of along and narrow shape for that purpose. This magnet is placed on a sliding carriage,by means of which it is made to approach the specimen or recede from it with greatfacility and in the most gentle manner. In consequence of finding decided polarity