809 



would continue to be propagated in the great circle coinciding with 

 its first direction, unless accidental causes should alter its course. 



VIII. On the existence of a Zone of least disturbance in the Shell. 



The author investigates analytically the position of this zone, and 

 from the results of his investigation, points out the conditions under 

 which it will exist, and also the consequences that will follow from 

 its non-existence. 



IX. This section is devoted to the calculation of some of the con- 

 stants contained in the formulae of the preceding sections. 



The following are the geological deductions from the foregoing 

 investigations : — 



1. The stability of the axis of rotation of the earth will progress- 

 ively increase during the process of solidification. 



2. By employing the values of the constants obtained in Section 

 IX., it appears that the thickness of the earth's crust cannot be less 

 than 18 miles, and cannot exceed 600 miles. 



3. The earth's primitive ellipticity, when entirely fluid, was less 

 than its present ellipticity ; but their difference may be neglected. 



4. If a zone of least disturbance existed near the parallel of mean 

 pressure, the directions of great lines of elevation should be in ge- 

 neral parallel, or perpendicular to the equator. Its non-existence 

 there, which observation seems to show, proves at least that the va- 

 riable pressure did not predominate over the constant. Since, as yet, 

 observation goes to prove that such a zone does not exist on the 

 earth's surface, we must provisionally conclude that the constant 

 pressure greatly predominated over the variable, and, consequently, 

 that the directions of the lines of elevation must be comparatively 

 arbitrary. 



5. That great friction and pressure exist at the surface of contact 

 of the nucleus and shell, is shown from the conclusions arrived at in 

 Section IV., combined with the important result obtained by Mr. 

 Hopkins in his second memoir on Physical Geology (Phil. Trans. 

 1840, p. 207). 



6. The amount of elastic gases given off from the surface of the 

 nucleus rapidly decreases as the thickness of the shell increases. 



7. The expression obtained for the variation of gravity shows that, 

 if the angular velocity of rotation of the earth remained unchanged, 

 the waters on its surface would tend to accumulate towards the 

 equator, for the increase of gravity, in going from the equator to the 

 poles, would be less according as the shell's thickness increased. 



March 22. 1849. 



The Very Rev. The DEAN OF WESTMINSTER, Vice-President, 

 in the Chair. 



A paper was read, entitled "An Account of the Aurora Borealis 



