346 



POPULAR SCIENCE MONTHLY 



second and so on — a well-known experiment, made every da}- for ele- 

 mentary classes. Xor is this all, for he interposes between his lode- 

 stone and a mass of iron thick boards, walls of pottery and marble, and 

 even metals, and he finds that there is nanght so solid as to do away 

 with this force or to check it, save a plate of iron. All that can be added 

 to this pregnant observation is that the plate of iron mnst be very thick 

 in order to carry all the lines of force dne to the magnet, and thus com- 

 pletely screen the space beyond. 



Bnt Gilbert is astonishing when he goes on to make thick l)oxes of 

 gold, glass and marble, and suspending his needle within them, declares 

 with excusable enthusiasm that regardless of the box which imprisons 



Fig. 2. Gilbert heated and hammered Bars of Iron in the Magnetic Meridian, and 



ALLOWED THEM TO COOL WHILE LYING NORTH {Scptetltrio) AND SOUTH (Auster). 



the magnet, it turns to its predestined points of north and south. He 

 even constructs a box of iron, places his magnet within, observes its be- 

 havior, and concludes that it turns north and south, and would do so 

 were 'it shut up in iron vaults sufficiently roomy.' Our experiments 

 show that if the sides of the box are thin, the needle will experience 

 the directive force of the earth ; but if they are sufficiently thick — thick 

 as the walls of an ordinary safe — the inside of such a box will be com- 

 pletely screened; none of the earth's magnetic lines will get into it so 

 that the needle will remain indifferently in any position in which it is 

 placed. A few years ago, the physical laboratory of St. Johns College, 

 Oxford, was screened from the obtrusive lines of neighboring dynamos 



