and certain Electrical Phenomena. 479 



granite block, weighing above sixty tons, is so nicely balanced 

 that the strength of one man is sufficient to make it oscillate. 

 He explains how granite disintegrates by weathering into 

 rhomboidal and tabular masses, which by the further opera- 

 tion of air and moisture gradually lose their solid angles and 

 approach the spheroidal form. The upper part of the cliff 

 being more exposed to atmospheric influences than the parts 

 beneath, it happens that these rounded masses frequently rest 

 on tabular blocks ; and as the lesser axes of these rude 

 spheroids are perpendicular to the horizon, it is conceivable 

 that some of them may be made to rock on their point of 

 support. 



This subject was further investigated by Mr. Justice Grove, 

 F.R.S., in a paper which I heard him read in Section of 

 the British Association at its meeting at Norwich in 1868. 

 The following is a very short abstract of his paper. If we 

 suppose a slab of stone lying on another, both having flat 

 surfaces, the attrition and disintegration produced by changes 

 of weather, of temperature, &c., would act to the greatest 

 extent at the corners, and next to them at the edges, because 

 these parts expose respectively the greater surfaces compared 

 with the bulk of the stone. This would tend to round off all 

 the angles and gradually change the rhomb more or less 

 towards an oblate spheroid. If w T e assume the w r earing away 

 between the stones to reach a point which is not in the line 

 of centres of gravity, the upper stone would fall on one side, 

 leaving the unworn point most exposed to be acted on by the 

 water lying in the angle of the crevice, evaporation being 

 less rapid there than at other parts. This point w r ould then 

 be worn away, and the stone would fall back a little, then 

 fresh action upon new surfaces, another oscillation, and so on. 

 Thus, by insensible progression, it becomes not improbable 

 that the last point or line worn away would be the point or 

 line on which, from its being on the line of centres of gravity, 

 the upper stone would rock. 



In support of this view the following experiment was 

 tried : — Two parallelopipeds of iron were placed one on the 

 other in dilute acid, which acted on them for three or four 

 days, when the upper one was found to be a perfect analogue 

 of a rocking stone so delicately balanced on two points that 

 it could be made to rock by blowing on it with the mouth. 

 It was observed that the iron rocked only in one direction. 

 Such is the case with the great Logan Stone, and, probablv, 

 with most other rocking stones, since it is more likely that 

 a stable equilibrium should be attained on two points than on 

 one. If the surfaces of the stones be in such close contact 



