NATURAL PHILOSOPHY. 151 



placed upright or proximately so, had its polarity reversed by turning it 

 with the contrary ends downwards, and again became neutral when placed 

 on the horizontal east and west line. If the same bar, while held in an 

 upright position, or inclined in the axil direction of the earth's magnetism, 

 were subjected to percussion, or other mechanical violence, not only did 

 its magnetism become much more powerful than that of simple induction, 

 but it strongly exhibited its augmented polarity when placed in the east 

 and west neutral or equatorial position, and, however it might be reeved 

 about or swung round, its polarity remained the same. Having proved 

 these two propositions by experiment, Dr. Scoresby went on to apply 

 them to the case of iron ship?, and to point out that, in consequence of 

 the percussive action to which the material was exposed while the ships 

 were in course of construction, it became as intensely magnetic as it was 

 possible for malleable iron to be. This augmented magnetism, however, 

 was not permanent or fixed, but, under different circumstances as to the 

 relative deviation of the ship's magnetism and that of the earth, was easily 

 changeable, and liable necessarily to be changed. The magnetism devel- 

 oped by mechanical violence could readily be neutralized or changed, 

 under a proper change of conditions, by other processes of mechanical vio- 

 lence. If the bar of iron magnetized by hammering were held in the 

 reverse direction from that in which the magnetism had been developed, 

 and again hammered, the polarity would not only be altered, but reversed. 

 Again, after well hammering the bar in a vertical position, let it be quickly 

 reversed, the lower end, as hammered, now being upwards, and let one of 

 its extremities be then presented to a delicate compass ; the deviating 

 influence in this case would be but small, perhaps a few degrees only, from 

 the influence of the earth's magnetism, it being now augmented by the 

 increased magnetism of the bar. If, while held in this position, a single 

 blow were struck on the bar with a hammer, the needle would be seen to 

 fly round as if by magic, and settle at a point of deviation perhaps four or 

 six times as great as before. The result of another experiment which had 

 been made with elongated plates of iron, to elucidate the phenomena of 

 mechanical violence or vibration, had been still more remarkable. Take a 

 couple of iron plates laid together ; it was found when their condition was 

 neutral, that held within two or three inches of a compass, horizontally 

 east and west, there was no action whatever on the needle ; but, holding 

 the plates upright and bending them, or shaking them with the hand, or 

 merely giving them a vibratory shake, and then presenting them as before 

 to the compass, the iron was found to have become very strongly magnetic, 

 the end which was downward repelling the north pole of the needle. 

 Reversing the position of the plates, while held upright, let the vibratory 

 action be repeated, and the end formerly repelling will now be found to 

 attract the north end of the pole. Repeating the vibratory action, while 

 the plates Avere held horizontally in an east and west line, the magnetism 

 would be found, on bringing the plates to the test, to have disappeared, all 

 action of the compass haAdng gone. To meet probable objections, he had 



