TERRESTRIAL MAGNETISM, ETC. 285 



attached to each card ; in the compass described 

 below there are four pairs of comparatively very 

 small needles. 



Instead of the mere paper or pasteboard 

 described by Gilbert, a thin disc of mica, with 

 paper pasted to it on each side is used for the fly- 

 card, as rendering it less liable to warp. The cir- 

 cumference of the circle is divided to degrees, and 

 the thirty-two points of the ordinary compass are 

 shown by bold marks a little inside the circle of 

 degrees, as pictured in the reduced copy of a com- 

 pass card at page 228. A jewelled cap fixed in the 

 centre of the card bears the whole weight of the 

 card and needles on a fine point of hardened steel 

 or of a natural alloy of iridium and osmium (which 

 is also used for the points of gold pens), being a 

 substance much harder than steel, and not like 

 steel liable to rust. 



The proper size for the compass card is a subject 

 on which there has been great diversity of opinion 

 and diversity of usage apparently from the begin- 

 ning. Gilbert, in describing the azimuth compass 

 of his own invention, specifies " at least a foot " as 



