Influence of Terrestrial Magnetism on Iron. 209 



from terrestrial magnetism, the influence of which is favoured 

 by this action. 



To the influence of this magnetism must be attributed the 

 magnetisation possessed by all magnetic bodies left for any 

 length of time in the same place ; thus, the rods of lightning 

 conductors, the points of steeples.* bars, or other iron ob- 

 jects, placed in buildings, always present traces of magnet- 

 ism ; it is the same with iron or steel tools, such as those of 

 a locksmith ; or punches, or cutting instruments that are 

 liable to undergo vibratory movements by the use to which 

 they are applied. We can even obtain powerful magnets, 

 from the magnetism produced by means of the terrestrial 

 globe, by taking a certain number of iron wires, twelve or 

 fifteen inches in length, and twisting them strongly while 

 held in a vertical position, or which is better still, in the di- 

 rection of the dipping needle. This operation, which renders 

 them stiffer, facilitates, at the same time, the development 

 within them of a very powerful magnetism ; and, when once 

 they have been magnetised, they are united to form a bundle, 

 care being taken that their similar poles are all at the same 

 extremity of the bundle. 



The magnetising action exercised by terrestrial magnet- 

 ism upon iron may determine upon the needles of compasses 

 very serious deviations, when they are placed upon vessels 

 in motion. In fact, these structures which always contain 

 in their fabric a very considerable quantity of bars and plates 

 and rods of iron, are found, from this circumstance, to con- 

 tain magnets ; the poles of which must change with the po- 

 sition of the vessel, in respect to the magnetic meridian. 

 There is produced, therefore, upon the magnetic needle, a va- 

 riable action, the effect of which it is impossible to determine 

 beforehand ; whilst, if the vessel always remained at the 

 same place, it would be an easy matter to appreciate the in- 

 fluence of this cause of deviation, and to take account of it. 



* It is probable that, in respect to elevated iron points, such, in particular, 

 as those of lightning conductors, atmospheric electricity, or more especially 

 that from lightning, contributes its part to their magnetisation as much, and 

 more so. than terrestrial mncrnetism. 



