536 Correspondence — Rev. 0. Fisher. 



method ; although T saw that they did not establish the opposite to 

 his conclusion, which was, that the solid crust of the earth cannot be 

 much less than 1,000 miles thick.^ Now, however, the Archdeacon 

 comes to the rescue with the communication given in the September 

 Magazine. What he proves appears to me to be this, — that if the 

 crust of the earth is devoid of rigid connexion with the nucleus, 

 and if a force acts upon it so as to give it a motion of rota- 

 tion different from that which the nucleus already has, (in the case 

 before us, the new rotation is given about a diameter of the equator, 

 say that which momentarily coincides with the equinoctial line,) 

 then the new rotation given to the crust will not at first be com- 

 municated to the nucleus. I say devoid of rigid connexion, for the 

 effect of such connexion as arises from viscidity, or friction, or any 

 similar cause, will simply be to communicate the new rotation to the 

 nucleus less gradually than if the nucleus was absolutely fluid, and, 

 therefore, the connexion nil. But in any case short of rigid con- 

 nexion, the new rotation will not be communicated to the nucleus at 

 all, at first. I mean at the moment at which it is communicated to 

 the crust. Some time will be necessary for that result. 



Now, the "processional force," as the Archdeacon calls it, is " ever 

 alive and active ; " that is to say, we are obliged to consider at every 

 moment its effect at that moment, or at first. And that effect is, to 

 be producing at every moment a rotation in the crust, which will not 

 have time to communicate itself to the nucleus before, by combining 

 with the pre-existing rotation of the crust, it has caused the preces- 

 sion of the pole. The result will be that the amount of precession 

 of the pole will be different in the case in which the nucleus is not 

 rigidly connected with the crust, from what it would be if it were so. 

 This is the conclusion that Archdeacon Pratt insists upon, and it 

 seems certainly a just one. But, nevertheless, I cannot but think 

 that the viscidity must have some effect upon the precession ; and I 

 should be very glad if your Correspondent would explain of what kind. 

 What seems to me the case is this. The effect of the processional force 

 on the crust is to set it rotating about a generating line of the cone 

 in which the axis moves, always a little in advance of the line round 

 which the nucleus is rotating, and the viscidity will cause the axis 

 of rotation of the crust to drag that of the nucleus after it. Will not 

 this dragging effort have the effect of retarding the rotation of the 

 crust in an extremely small degree, so that in the case of a viscid 

 nucleus, the effect of the action of the sun and moon on the pro- 

 tuberant matter, as the equator is, in truth, indirectly to lessen the 

 angular velocity of the earth, contrary to what is stated to be the 

 case upon the usual supposition of the earth's I'igidity ? 



If the rotation of the earth is diminished gradually, the precession 

 will be increased accordingly. 0. Fishek. 



Harlton, near Cambridge. 



1 See my paper on the elevation of mountains, &c. Cambridge Philosophical 

 Transactions, vol. xi., part iii., p. 2. See also Geol. Mag., 1868, Vol. V., p. 493. 



