TERRESTRIAL MAGNETISM, ETC. 287 



better it ought to be chosen, not only for azimuth 

 or standard compasses, but also for the steering 

 compass, on which immediately depends the 

 straightness of the ship's course, a result of para- 

 mount importance. But, in fact, taking compasses 

 as ordinarily made hitherto, the smaller compasses 

 do not work nearly so well as the larger. With 

 similar care as to the bearing-point and cap, a ten- 

 or twelve-inch compass, while more accurate or not 

 less accurate in respect to error arising from friction 

 on the bearing-point, is much steadier in a heavy 

 sea than a compass of six or seven inches diameter ; 

 and it is, in reality, practical experience of this 

 advantage, not merely convenience of the larger 

 card for reading azimuths on it or for steering by 

 it, that has led to the general preference of ten- 

 inch compasses in the British merchant service. 



The secret of the steadiness of a large compass 

 is the longness of its vibrational period, and a small 

 card would have the same steadiness as a large one 

 if its vibrational period were the same. How little 

 this is known is illustrated by the methods of pro- 

 curing steadiness in common use. In some (as in 



