•294 Sir George Greenhill [May 3, 



have been offended when his apparatus was found more useful still in 

 increasing the rolling, and maintaining it, in this case of an ice- 

 breaker, to worry a way easier through the pack ; or even in working 

 off a sandbank. A different setting of the buffer valve was all that 

 was required. 



A spinning-top stands up vertical in a smooth cup, even when the 

 cup is moved about, as on a rolling ship, and as I move the Maxwell 

 top here ; so that if the top carries a polished mirror it can serve as 

 the mercurial horizon does on terra firma, and so give an altitude 

 when the sea horizon is obscured. The idea was suggested by Serson 

 in 1744, and the enterprising Admiralty of that day did not crab the 

 idea straight off with the usual " won't work," but sent him-to sea 

 in the "Victory" to make a practical test; unfortunately the ship 

 was lost with all hands on the Casquets near Alderney. A specimen 

 from the King's collection is preserved in King's College, Strand. 

 The idea has been revived of late years by French navigators, as 

 the Fleuriais gyroscopic horizon ; it is claimed to give good results, 

 in skilful hands, where an ordinary observation would be imprac- 

 ticable through fog. 



But the most important service to navigation in recent times of 

 the top in harness is the gyroscopic compass. The idea was suggested 

 by Sir William Thomson, but the high spin requisite could not be 

 realized in his day till the great improvement arrived in modern 

 mechanical skill of an Anschutz or Sperry, as a steel fly-wheel was 

 required, some 4 inches in diameter, spun at 20,000 revolutions a 

 minute. The axle, mounted freely, is always striving after the 

 position as close as it can get to the direction of the polar axis, and 

 so carries the compass-card with it j)ointing due north, with no 

 magnetic variation requiring constant correction. 



Because in modern swift steam navigation across the Atlantic, 

 where the great circle course must be maintained, practically the only 

 nautical observation required is for azimuth, in its correction of 

 magnetic variation ; and there is no variation in the gyro-conipass. 

 A specimen would be too complicated and delicate to show off in 

 this room. And if any young researcher should take it into his head 

 to test the action by pushing the card away from its course, it would 

 take an kour or more to swing back into place again. 



But the greatest spinning-top we know is this Earth itself, spinning 

 round once a day, with the axle pointing at the Pole Star. Ancient 

 astronomical observation reveals a precession, as in this ^laxwell 

 top, twirled with the left hand, so that the pole is making a circuit of 

 the sky, which will be completed in 26,000 years. Since Homer's 

 day the pole has made more than one-tenth of the way round, and 

 the constellations have changed from one sign of the Zodiac well 

 through the next, and back beyond. We are able, then, to assign 

 a date to Homer and Hesiod from their astronomical allusions. 

 Thus the nymph Calypso gives Ulysses his final instruction in 



