BRITISH ASSOCIATION, &C. 143 



this process, unless there be some interfering action, it becomes not im- 

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

 which, from its being in the line of centres of gravity, the upper stone 

 would rock. After seeing the great Logan- stone near the Land's End, I traced 

 so many other approximations to rocking- stones along that coast, that I 

 became satisfied, as far as one ought to be satisfied on any subject of human 

 enquiry, that this was a correct theory. It then occurred to me, if this view 

 be true, may we not be able to hasten the operations of nature so as to 

 produce artificially (if such a term may be used) the rocking- stone results. 

 A very little thought suggested the experiment. Two parallelopipeds of iron, 

 which had been made for keepers of magnets, were taken similar, but that 

 one was twice the length of the other. The shorter was superposed on the 

 longer, and both immersed in sulphuric acid diluted with three times its 

 volume of water, some nitric acid being added at first to hasten the corrosion. 

 The liquid was changed from time to time as it became nearly saturated, 

 but without changing the position of the iron. At the end of three or four 

 days the pieces of iron were taken out, washed, and examined, 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. [Eesult shown.] It will be observed in this experiment that the 

 iron rocks only in one direction. Such is the case with the Great Logan- 

 stone, and I believe with the greater number of rocking-stones. It is ob- 

 viously more probable that a stable equilibrium should be obtained on two 

 points than on one. I have not yet got a specimen to rock or spin upon one 

 point. [Approximation to this shown by two zinc discs and explained.] If 

 the surfaces of the slabs be in such close contact that there is not room for 

 circulation of the saturated liquid, a formation like those near the Cheese- 

 wring will be effected, or if a number of discs or slabs be superposed and the 

 lower ones more exposed to the weather, and to catch the dripping and 

 drifting water from the upper, we should get a formation exactly like the 

 Cheesewring, which may be called an incipient-v.compound rocking-stone, in 

 that each slab is worn away at the edges, and the lower ones much more 

 than the upper, so that if left alone, v,'hich it won't be, and if it does not 

 topple over too soon, which it probably will, it might well end in a rocking- 

 stone. I should not be surprised if it rocked now in a great storm. 



Section F. — Economic Science and Statistics. 



After the reading of a Paper, by Dr. Ceisp, on the " Statistics of Pul- 

 monary Consumption in 623 Districts of England and AVales," Me. Holland 

 said he was employed some years ago by Government to investigate the cause 

 of an alleged excess of phthisis in the Cornish districts. In the Eegistrar- 

 General's returns all pulmonary diseases causing consumption were classed 

 under the head of phthisis. Now, this was a great mistake, consumption 

 and phthisis being often quite different things. Upon investigation, he found 

 that the lives of the Cornish miners were shortened ten years on the aver- 

 age by pulmonary diseases, but that the proportion of phthisis was not in 



