VOL. LXIX.] PHILOSOPHICAL TRANSACTIONS. SQl 



lower extremity of a steel magnet, placed in a vertical position, and highly po- 

 lished. This whole apparatus was placed in a glass cylindrical vessel, filled with 

 water, so far as to cover the point of the working needle. The horizontal 

 magnetic needle, thus suspended, was remarkably quick in directing itself into 

 the magnetic meridian, and was very easily displaced out of its direction by the 

 approach of a magnet at a considerable distance. The greatest difficulty found 

 in this contrivance was, that a little jerk shook the point of the sewing needle 

 from the vertical magnet ; and that a heavy magnet could not be suspended from 

 the point of a sewing needle, but required a tolerably good magnet rounded at 

 its extremity, where it was suspended from the vertical magnet, or a thick piece 

 of iron rounded in the same manner, by which means, however, the horizontal 

 magnet did not move so freely. 



Exp. 5. He adapted to a fiat strong magnetic needle, 2 caps, turned one 

 against another, so that the needle could be supported on either of the caps, and 

 turned with either surface upwards. He fixed on one of the flat surfaces, on 

 each side of the centre, a thin glass tube, hermetically sealed, of such a size, 

 as made the whole together dip in the water so deep that only a small part of the 

 glass tubes remained above the surface of the water. He then depressed the 

 needle entirely under the surface of the water, by thrusting a metallic point in 

 the cap which was uppermost, and fixing this point to the cover of the basin : 

 thus the needle, kept under water, bore against the point only with that small 

 degree of pressure which the very inconsiderable difference of specific gravity, 

 which it had above an equal bulk of the water, could give it, which did not 

 amount to above a few grains of weight. Thus this needle, losing nothing of its 

 polarity, lost very near the whole of the resistance of its weight, and at the 

 same time that quivering motion and restlessness, which it had in the air, moving 

 smoothly in a medium, which could only obstruct its too great vibrations (almost 

 in the same way as astronomers stop the vibrations of their plumb-line by hang- 

 ing the weight in water or oil) without obstructing (but retarding only) its ten- 

 dency to the magnetic meridian, liquids pressing equally on all sides. 



Having now found, by experience, that a strong magnetical needle pointed to 

 the magnetic meridian near as well under water as in the open air, and that by 

 the resistance of the medium much of its too great versatility was taken away ; 

 Dr. I. wanted to try what degree of magnetism could be given to a thin steel 

 cylindrical tube, as a needle made in this shape could be as light as required, 

 without being encumbered with cork or glass tubes ; but finding no such tubes 

 ready made, he tried one made of tin, and found it susceptible of a much 

 greater magnetic virtue than he expected, considering that the iron plates of 

 which it is made are neither sufficiently hardened for this purpose, nor approach 

 enough to the nature of steel ; besides that, being covered with a pewter coating. 



