Manchester Memoirs, Vol. xlvi. (1902), A'^. 10- 5 



is marked in a vertical position, on the central part of the 

 screen, the horizontal motion of which across the parallel 

 band of light, manifests the axial rotation of the earth 

 directly as a sense impression. The rate of rotation of 

 the black band for the latitude of Manchester is two-tenths 

 of a degree per minute =30 hours per revolution of 360°, 

 36o°-^24 hours =15° ( Lat. 53° 29' sine = -804 X 15°= 12° 

 per hour nearly = 360°-;- 12° = 30 hours). 



Correlated with the doctrine of the diurnal rotation of 

 the earth is Halley's hypothesis of the differential rotation 

 of its internal parts about the same axis, to account for the 

 secular variation of the mariner's compass.* From obser- 

 vations of the variation, the illustrious astronomer inferred 

 that the direction of the internal motion was westward, 

 and, consequently, that the rotation of the inner sphere was 

 slower than the external shell of the earth. From further 

 observations, he concluded that the internal sphere made 

 one revolution backwards in 700 years. 



Halley was quite alive to the objection that would 

 be taken to his hypothesis, and anticipated it by 

 instancing the ring of Saturn, as showing a continuous 

 body rotating concentrically with a planet and having an 

 interval of space between them. He further remarks that 

 if this ring were turned on one of its diameters, it would 

 then describe such a concave sphere as he supposed the 

 external shell of the earth to be. Had Halley been living 

 at the present time, he would not have failed to find further 

 confirmation of the truth of his theory from the equatorial 

 regions of the sun rotating at a different velocity from that 

 of the regions nearer the poles, as observed by Carrington 



* Phil. Trails.^ Vol. xvii. 1692. Abridgment, Vol. iii., p. 470. 



