248 POPULAR LECTURES AND ADDRESSES. 



The smallness of the magnetic indicator here allows 

 the magnetic force to show its effect with compara- 

 tively little disturbance from gravity. The nature 

 of the magnetic action of the terella is further illus- 

 trated by Gilbert in the annexed diagram (Fig. 33), 

 reproduced in facsimile from his original edition. 1 

 It represents the directions taken by a small 

 magnetized steel needle, supported by a cap on a 

 finely pointed stem, at different positions in the 

 neighbourhood of a terella. The same results 

 are shown more completely and more accurately 

 by the diagram of curves shown in Fig. 34, 

 which have been calculated mathematically from 

 the laws of magnetic force discovered by Coulomb 



1 In page 14 of Lochman's edition there is a curious error in 

 this diagram, which is repeated in page 80, the needle in the 

 equatorial position being shown with the arrow-head intended to 

 denote its true south pole turned towards the south instead of 

 towards the north of the terella. Lochman's wood-engraver 

 generally reversed Gilbert's diagrams as to right and left (giving, 

 for example, a remarkable picture of a blacksmith wielding with 

 his left hand a hammer to strike a piece of iron on the anvil, as a 

 reproduction of Gilbert's picture which shows a blacksmith working 

 with his right arm), and seems to have corrected the reversal for 

 two of the needles, and omitted to do so for the other, in his 

 diagrams of the terella. 



