68 Royal Society. 



VII. The directiojis of the Fissures in the Shell which might be prO' 

 duced by the action of the pressures in Section I. 

 The author states that the tendency of the variable pressure is in 

 the first instance to produce fissures parallel to the equator ; that 

 when such a fissure was once commenced the tendency would be to 

 propagate it along a parallel of latitude, until the force of the ten- 

 sions became sufficiently lessened by the separation of the extended 

 portion of the shell ; and that similar fissures would be formed si- 

 multaneously and symmetrically on each side of the equator. Sub- 

 sequently, as may readily be deduced from Mr. Hopkins's investiga- 

 tions, the tendency will be to form fissures at right angles to those 

 previously existing. If, however, the constant pressure were far 

 greater than the variable, the directions of the fissures would be go- 

 verned chiefly by accidental causes ; but if a fissure commenced, it 

 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. 



