34 SECTION PHYSICS. 



aluminium rim which is covered with a piece of paper gummed to it 

 in order that the compass card itself may be gummed on to that. The 

 card consists simply of a single thickness of paper, and I find it 

 necessary in the larger sizes to cut the paper radially, because when it 

 becomes heated by the sun it shrinks and tends to warp the rim into a 

 saddle shaped surface. That is obviated by radial slits in the paper. 

 The magnets are hung direct to the rim, and this specimen before you 

 is the first in which there is an arrangement for a perfect adjust- 

 ment of the magnets. I have got them out of position by throwing 

 it about, but I can adjust them in a quarter of a minute, and before 

 going to sea a little shellac could be applied to make the position 

 permanent. The magnets are placed in a row like a rope ladder, 

 being fastened by half hitches of silk thread around each magnet, like 

 the ratlins of a rope ladder. By the aid of a straight edge I can very 

 easily adjust the magnets exactly to the north and south line. The 

 magnets are hung on the rim by a silk thread and lastly the rim is 

 supported on the central boss by a silk thread also, that being put on 

 under equal tension in such a way as to secure exact and constant 

 centreing. The object is to get needles in a compass which will work 

 well at sea, and for this purpose it is found that a certain slowness of 

 period is necessary, accordingly I have a forty second period with 

 this, and this will be as steady at sea in air as any compass can be, so 

 that liquid compasses will be unnecessary. The other one has a 

 period of thirty three seconds, and the smallest has the same period 

 as the Admiralty compass, and therefore will have approximately the 

 same steadiness at sea. 



With regard to the other correctors, I shall only say that a perfect 

 system of adjustment for different latitudes must be carried out. The 

 Astronomer Royal long ago pointed out this, and many patents have 

 been taken out for various methods of adjustment which have fulfilled 

 the conditions laid down by the Astronomer Royal with more or less 

 practical availability. I have endeavoured to introduce a system of 

 adjustment which would be more absolutely and perfectly simple and 

 ready in its use. I should have had great pleasure in shewing you a 

 binnacle with this compass in it, but by some mischance it is not 

 here. It is, however, in London, I believe, and you will have an 

 opportunity of seeing it from to-day forward. The variable effect clue 



