THE GUIDING-NEEDLE ON AN IRON SHIP. 299 



■wheels, and weights by that mysterious agency we call magnetism, 

 and the oscillations of the magnet, when drawn out of parallelism 

 to the lines of force, will be entirely similar. Such are the efforts 

 of the compass to regain its normal direction 

 when disturbed ; and the test of a good com- 

 pass is the sensitive quickness with 

 which it will turn aside from the 



j^. 



,z. 



M 



yy/y. 



y/'/!''//'','',''''''//.>'. 





magnetic meridian when another 

 magnet is brought near, and 

 the celerity of its return 

 thereto when the in- 

 truder is removed. 



Fig. 8 —Mechanical 



Illustration of the 



Magnetic Force that 



GIVES THE CoMPASS-NKEDLB 



Direction. The dotted parallel 

 lines represent the earth's mag- 

 netic lines of force. 



The Iron Ship a Mag- 

 net. — It is a characteristic of 

 ,'>;';:;<-;.'•'•'" every mass and particle of iron on the 



'//''' earth's surface to acquire in varied de- 



gree the terrestrial magnetism that sur- 

 rounds it ; and this agency enters naturally, 

 without effort or force : it is gently induced in the 

 material so congenial to it, by the mere fact of the 

 material quietly lying in its midst — the magnetic 

 field, which pervades all space. And the word iron is not here 

 used in a specific sense, but as a general term to include wrought- 

 iron, cast-iron, and steel, which are all susceptible to magnetism. 



The steel rails that afford transit from seaboard to interior, the 

 trestle-work upon which the elevated trains traverse the metrop- 

 olis, the heavy castings in a foundry, the massive forgings in a 

 machine-shop, even the little scraps upon a neglected heap, have 

 one and all magnetic features that distinguish them from other 

 metals, and point out the common kindred among themselves. 

 And these features are entirely analogous to those of the steel 

 magnet already described — two poles, one at each end of the mass, 

 with a neutral belt between. 



Let us conceive a metallically pure cylinder of wrought or 

 cast iron that has not been hammered, and let us further conceive 

 it entirely free from magnetism : hold it vertically, and instantly 

 the upper end becomes a south, and the lower a north pole (in 

 this latitude). Reverse it as quickly as we may, and the magnet- 



mm 





