88 Prof. Challis's B£searches in the 



parts. A piece of attractive glass, 27 millims. in length, 

 which was suspended between tlie polar faces, 29 millims. 

 apart, in such a manner that the extremities of this needle 

 were not more than 1 niillim. distant from the polar faces, 

 was made to oscillate each time during 30 seconds. At an 

 equal distance from the upper and lower edges it made only 

 45 oscillations in the 30 seconds, but level with the polar faces 

 it made nineteen. 



When the polar faces are at this distance, the needle does 

 not assume the equatorial position when it is suspended above 

 their edges. At the distance of 4*5 millims., it made 5'5 

 oscillations; 13*5 millims. distant it made only 2"5 oscil- 

 lations. The polar faces were approached to within 3 millims. 

 The needle, which now could not assume the axial position 

 between the faces, nevertheless showed its tendency to take 

 that position ; but raised 2 millims. above their edges, it 

 assumed the equatorial position, and made eighteen oscilla- 

 tions in 30 seconds. At a distance of y^^ths of a millim., it made 

 thirty-five oscillations. At the least possible distance, so as just 

 to avoid contact with the polar pieces, it made forty-five 

 oscillations. 



It is seen that both the repulsive as well as the attractive 

 diamagnetic bodies make more numerous oscillations in a 

 parallel position to the polar faces than in the perpendicular 

 position. It must however be observed, as has already been 

 done, that the determinations of the numbers have not yet 

 been carried sufficiendy far to serve for the calculation of their 

 laws. 



I lately made some experiments on the influence which heat 

 exercises on diamagnetic substances. These experiments are 

 not yet numerous enough ; they however prove to me that 

 some attractive diamagnetic substances pass into the class of 

 repulsive diamagnetic substances by an increased temperature. 

 The only substance which exhibits this effect in a high degree 

 is brass. My analogous experiments on other bodies are not 

 yet sufficiently decisive to warrant their publication. 



XI. Continuation of Researches in the Mathematical Theoi-y 

 of Aerial Vibrations. By the Rev. J. Challis, M.A., 

 F.R.S.i F.R.A.S, Plnmian Professor of Astronomy and 

 Experimental Philosophy in the University of Cambridge'^ . 



IN replying to arguments against my conclusion that plane- 

 waves are physically impossible, I am required by the rules 

 of logic to notice those only which profess to invalidate any 

 * Communicated by the Author. 



