Schaw. — On the Last Glacial Epoch. 525 



parallel in the case of Saturn, encouraged this idea. La Place 

 and others had calculated that the plane of the earth's orbit 

 could not vary more than a certain small amount, — in which 

 they did not all agree, — but no one had discovered how much 

 it did move, or where the pole of the ecliptic would be at any 

 particular epoch. Nor had any one attempted to determine 

 what effect the known movement of the zenith of the north 

 pole of the earth has upon the zeniths of other places on the 

 earth. 



As an artillery officer he had studied the gyroscope with 

 reference to the peculiar movements of the spinning projectiles 

 thrown by rifled ordnance, and he knew that a rotating body, 

 if perfectly balanced, maintained the direction of its axis per- 

 fectly in the same direction ; but he also knew that a very 

 small deviation of the centre of gravity from the centre of 

 form produced a slow second rotation by which the direction 

 of the axis of primary rotation was gradually altered. The 

 question then arose in his mind, Is this what the earth is 

 doing ? Is it also slowly rotating on some other axis, so 

 that the poles of the axis of primary rotation are each 

 describing circles round the poles of this secondary axis ? If 

 so, the first point was to find the pole of this axis of 

 secondary rotation. The data he had to work from were 

 the recorded positions of a number of the principal stars, and 

 also the position amongst the stars of the north pole at certain 

 dates extending back for some 2,000 years — from the star cata- 

 logue of Hipparchus, dated 140 B.C., and that of Ulugh Beigh, 

 dated 1463 a.d., down to the more complete and accurate 

 lists of later years. He laid down the arc described by the 

 pole of the heavens between those dates, P 1 P 2 P 3 (Plate XLV., 

 fig. 1), and then by careful examination of the star lists he 

 found that a certain star r had not varied its distance from 

 the pole when the pole was at or near P 1 , while other stars 

 had varied their distauces ; he concluded that r was therefore 

 in the direction of the centre of the circle of which P 1 P- P 3 

 was an arc when the pole was at P 1 . Similarly, when the pole 

 had reached P 2 and P 3 successivelv, he found that certain stars 

 s and t respectively did not vary their distances from P 2 and 

 P 3 . The intersections of the lines PV, P 2 s, P% produced in 

 the point C, showed that C was the centre, round which the 

 pole of the northern heavens was slowly moving in a circle, 

 and this was not the pole of the ecliptic, but 6° from it. 

 Thus he obtained a sure standing-ground, and he proceeded to 

 develope the full consequences of this brilliant and important 

 discovery. 



But first he checked his conclusion as to the true centre of 

 the circle described by the north pole of daily rotation by 

 means of the geometrical truth that all angles in the same 



