Schaw. — On the Last Glacial Epoch. 529 



revolutions would be required to produce a second rotation if 

 a weight of 4-oz., or /o-nd part of the weight of the wheel, were 

 added to one side to throw it out of balance, as compared with 

 the 230 1 000 th of the mass of the earth which destroys its 

 balance and produces a second rotation in 11,680,000 revolu- 

 tions ; gV of weight of gyroscope : 230 1 0oo of weight of earth 

 : : 11,680,000 revolutions of earth = 1,625 revolutions of the 

 gyroscope, or 32 seconds. That is, under the above conditions 

 the gyroscope ought to complete a second rotation in a little over 

 half a minute ; and this is approximately what would happen. 

 Of course the comparison is very rough, because friction inter- 

 feres very greatly with the action of the gyroscope, and the 

 mode of the attachment of the extra weight and the direction 

 of the pull of gravity are different ; the estimate also of the 

 protuberance on the earth is very inexact : still, it is sufficient 

 to show that the second rotation of the earth is not only a 

 fact discovered and proved astronomically, but that an efficient 

 cause to produce this effect exists in the present distribution 

 of land and water on the earth. Possibly the inequality of 

 the earth's equatorial and polar diameters (to which Herscbel 

 in his popular explanation attributed the precession of the 

 equinoxes) may have a perturbing effect, as the inclination 

 of the earth's axis of daily rotation to the plane of its orbit 

 varies in its second rotation. 



There is, as I have said before, a wide field for future in- 

 vestigation opened up by this discovery of the second rotation 

 of the earth, and most probably it will be found that some 

 minor corrections must be made in some of General Drayson's 

 results, owing to perturbations resulting from other causes. 

 But the beauty and simplicity of the secondary motion of the 

 earth which has been brought to light by his sagacity and life- 

 long persistent labours seem to me to rank with the highest 

 discoveries of astronomy, and the name of Dray son will in the 

 future stand very high in the list of the great astronomers.* 



In connection with the ice age the historical nature of 

 General Drayson's discovery is of deep importance, and, 

 whether the great increase of the inclination of the earth's 



* This discovery will no doubt produce a revolution in astronomical 

 observatories, as the laborious observations now made year by year to 

 ascertain the apparent motions of the stars, and so by empirical rules to 

 calculate their apparent positions a few years in advance for the compila- 

 tion of the "Nautical Almanac," have become useless. The apparent 

 movement of each star, caused in reality by the second rotation of the 

 earth, can now be calculated with perfect accuracy for hundreds of years 

 in advance. A few competent men with calculating-machines can 

 easily perform what has hitherto needed a great staff of observers with 

 astronomical instruments, and subsequent laborious reductions of their 

 observations. Astronomical time, also, which has hitherto needed to be 

 fudged from time to time, will now be accurately determined. 

 34 



