122 ANNUAL OF SCIENTIFIC DISCOVERY. 



transverse to the direction of dip at Greenwich, or on a plane includ- 

 ing the direction of dip. In some experiments, these planes were 

 covered with flag stones, and the bars were laid upon the flag stones ; 

 in others, the bars were laid immediately upon the wood. While 

 there lying, they were struck v:l h iron or wooden hammers of differ- 

 ent sizes. The bars of the different classes were systematically inter- 

 mingled, in such a way that no tendency of the arms to give blows 

 of a different force or kind in special parts of the series could pro- 

 duce a class error in the result. For examination of the amount of 

 polar magnetism in each bar, ii: was placed at a definite distance (five 

 inches) below a prismatic compass, which was used to observe the 

 apparent azimuth of a fixed mark ; the bar was then reversed in 

 length, and the observation was repeated in that state. 



The number of experiments was twenty-one. They were varied 

 by difference in the succession of positions of the bars, difference of 

 time allowed for rest, difference in the violence of the blows, etc. 

 The principal results appear to be the following : 



1. The greatest amount of magnetism which a bar can receive 

 appears to be such as will produce (on the average of bars) a com- 

 pass deviation of about eleven degrees, the bar being five inches 

 below the compass. It was indifferent whether the bars rested on 

 the stone or on wood, or whether they were struck with iron or with 

 wood, the bars lying on the dip plane while struck. 



2. When the bars, thus charged, lay on the plane transverse to the 

 dip, they lost about one-fifth of their magnetism in one or two days, 

 and lost very little afterwards. 



3. When the charge of magnetism is smaller than the maximum, 

 the diminution in a day or two is nearly in the same proportion as for 

 the maximum. 



4. The effect of violence on the bars, when lying on the plane 

 transverse to the dip, is not in all cases to destroy the magnetism 

 completely ; sometimes it increases the magnetism. 



5. The cold-rolled iron receives (under similar violence) or parts 

 with (under similar violence) a greater amount of magnetism than 

 the hot-rolled iron, in the proportion of six to five. 



6. There is some reason to think that the hot-rolled iron has a 

 greater tendency to retain its primitive magnetism than the cold-rolled 

 iron has. 



7. There is some reason to think that, when lying tranquil, the hot- 

 rolled iron loses a larger proportion of its magnetism than the cold- 

 rolled iron loses in the same time. 



DEVIATION OF THE COMPASS AND THE MAGNETISM OF IRON SHIPS. 



At the meeting of the British Association, 1862, Mr. F. J. Evans, 

 on behalf of himself and others, read a report " On the Three Re- 

 ports of the Liverpool Compass Committee, and other recent Publi- 

 cations on the same subject," undertaken at the request of the 

 British Association. The papers included were, severally, by the 

 Astronomer Royal, the late Dr. Scoresby, and Capt. Johnson, R. N., 

 on the deviation of the compass and the magnetism of iron ships ; as 

 also contributions in the same field of inquiry by the reporters. After 



