296 POPULAR LECTURES AND ADDRESSES. 



write an article on the mariner's compass for 

 Good Words five years ago, and hence it is that 

 the article is not written until now. When there 

 seemed a possibility of finding a compass which 

 should fulfil the conditions of the problem, I felt 

 it impossible to complacently describe compasses 

 which perform their duty ill, or less well than 

 might be, through not fulfilling these conditions. 

 The accompanying diagram (Fig. 39) represents 

 the solution at which I have arrived. Eight small 

 needles of thin steel wire, from 3j inches to 2 

 inches long, weighing in all 54 grains, are fixed 

 (like the steps of a rope-ladder) on two parallel 

 silk threads, and slung from a light aluminium 

 circular rim of 10 inches diameter by four silk 

 threads through eyes in the four ends of the 

 outer pair of needles. The aluminium rim is 

 connected by thirty-two stout silk threads, the 

 spokes as it were of the wheel, with an aluminium 

 disk about the size of a fourpenny-piece forming 

 the nave. A small inverted cup, with sapphire 

 crown and aluminium sides and projecting lip, 

 fits through a hole in this disk and supports it 



