On the Periodic Movements of the Ground. 189 



simply halving the area and halving the distance, for we shall 

 then have made no change either in the magneto-motive force 

 or the resistances, and, consequently, the same total induction 

 is compressed into half the area ; or, again, we may double the 

 distance between the pole-pieces and preserve the same intensity 

 by simply doubling the length of the magnets. It is, in fact, 

 quite easy to design an arrangement of permanent magnets that 

 shall, within obvious limits, produce any given intensity of 

 magnetic field in any given area of cross-section of air-space. 

 These conclusions have been to a great extent tested by practice, 

 and so far as they go the analogy on which they are founded 

 holds good. The analogy is, in fact, mutatis mutandis, one of 

 universal application. 



The current by which the circuit is magnetized is always 

 applied when everything is in position ; otherwise very small 

 intensity results. The magnets are flashed by a current from a 

 dynamo to practical saturation. The coils are then removed, 

 and the magnets hammered severely. The hammering seldom 

 reduces the intensity more than about one per cent. In months 

 of continued testing I have never found any measurable falling- 

 ofF in the field of a magnet so treated. If a reversing current 

 is applied, the intensity will spring up on removal of the 

 current, and will sometimes continue to rise appreciably for 

 a considerable period. After a time it would probably fall 

 again ; but, if the weakening under the reverse current has 

 amounted to, say, twenty per cent., never to the lowest point 

 then reached, and, perhaps, never to the point to which it 

 springs at once, on the removal of the reverse current ; and, 

 further, I have little doubt that an experiment might be 

 arranged in which magnetizing and demagnetizing currents 

 should be applied alternately in such a manner that, on their 

 cessation, the successive strengthenings and weakenings of 

 the field would repeat themselves backwards, just as was the 

 case with the windings and unwindings in Sir W. Thomson^s 

 experiments on the " Fatigue of Metals." If so, the latest 

 experiments on torsion would but add another analogy to 

 Wiedemann's famous list. 



XXI. Note on M. Ph. Plantamour's Observations by means 

 of Levels on the Periodic Movements of the Ground at 

 Secheron, near Geneva. By Charles Davison, M.A., 

 Mathematical Master at King Edward^s High School, 

 Birmingham*. 



OBSERVATIONS on the movements of the ground by 

 means of levels have recently been made by many 



* Communicated by the Author. 



