178 Transactions. — Miscellaneous. 



from a naval point of view are numerous and important, and 

 more especially with regard to the delineation of the contour- 

 curves of the great ocean-basins, and the series of memoirs on 

 the oceanic circulation. The brilliant success of the " Chal- 

 lenger " expedition and its report gives us good ground for 

 hoping that one of Her Majesty's ships might be employed 

 in filling up some of the gaps which naturally occurred in the 

 explorations, and that, above all, some assistance should be 

 given to follow out the important lines of inquiry opened up 

 by the results of the soundings taken in the southern seas 

 in the neighbourhood of the Antarctic. As I have said on 

 another occasion, important magnetic and meteorological 

 problems demand investigation in the Antarctic, and I for 

 one would desire to see British sailors set out from this 

 British colony to once more force the icy gates of the South 

 and beard the ice-king in his solitary realms. 



Art. XIX. — A Comparison of the Magnetic Screening yro- 

 dticed by Different Metals. 



By J. A. Erskine, M.A. 



[Read before the Philosophical Institute of Canterbury, 6th November, 



1895.] 



When a conductor is placed in a varying magnetic field the 

 currents induced in it tend to keep the field constant. If the 

 field varies slowly the effect is slight ; but in fields produced by 

 rapidly-alternating currents the " screening" is very marked. 



In these experiments the fields were produced by leyden- 

 jar discharges. Magnetized steel needles were used as 

 "detectors" (Eutherford, Trans. N.Z. Inst., 1894, p. 488). 

 Magnetized steel needles are much more suitable for this pur- 

 pose than unmagnetized, for a field too weak to magnetize a 

 needle to any appreciable extent is capable of producing con- 

 siderable demagnetization. 



The discharge passed through a coil of several turns, 

 inside which a magnetized steel needle was placed, and in 

 whichever direction the needle lay it was partially demag- 

 netized ; but the demagnetization was greater when the 

 needle was placed in that direction in which the field, due 

 to the first semi-oscillation of the discharge, demagnetized it. 

 Hereafter this direction will be referred to as " direction a" ; 

 the direction in which the field due to first semi-oscillation 

 tended to magnetize the needle as " direction b." 



