TERRESTRIAL MAGNETISM, ETC. 293 



when east or west. A little later I shall have 

 occasion to explain the magnetic appliance for cor- 

 recting the heeling error, but when it is perfectly 

 corrected there remains a true dynamical rolling 

 error, which alone is enough both in wooden and 

 iron vessels, sailing or steam, to keep the compass 

 oscillating very wildly when the ship is rolling 

 considerably in a sea-way. 



When the free vibrational period 1 of the compass 

 card agrees with the period of the ship's rolling, a 

 comparatively moderate degree of rolling may 

 produce a great oscillation in the card. Now the 

 longest period of actual rolling, to any considerable 

 degree, in a sea-way is from fourteen to seventeen 

 or eighteen seconds. The vibrational period of 

 the " A" card of the Admiralty standard compass 

 is, in this part of the world, about nineteen seconds, 

 and that of the larger compass (ten-inch) of the 

 merchant steamers about twenty-six seconds ; and 



1 The free vibrational period, or simply "the period" (as it 

 may be called for brevity) of a compass, is the time it takes to 

 perform a complete vibration to and fro, when deflected hori- 

 zontally through any angle not exceeding 30 or 40, and left to 

 itself to vibrate freely. 



