50 Mr James Thomson on an Improved Tilting Apparatus 



of tlic size of the existing groups of atolls (excepting as many of tlie l)lgli- 

 est islands or mountains in such groups, as there usually occur of " en- 

 circled islands" in groups of atolls) ? and likewise what is the ordinary 

 height of the single scattered islands between such groups of islands ? — 

 subsidence sufficient to bury all these islands (with the abo-ve exception) 

 my theor}' absolutely requires, but no more. In my volume, I rather 

 vaguely concluded that the atolls, which are studded in so marvellous a 

 manner over wide spaces of ocean, marked the spots where the moun- 

 tains of a great continent lay buried, instead of merely separate tracts of 

 land or mountainous islands ; and I was thus led to speak somewhat more 

 strongly than warranted, of the probable vertical amount of subsidence 

 in the areas in question. 



Mr Lyell in the note alluded to, thinks we are much too irnorant of 

 intra-tropical geology (and ignorant enough we certainly are) to ;.ffirm 

 that calcareous rocks of the supposed thickness of coral reefs, do not 

 occur. I am inclined to lay considerable stress on this. I do not expect 

 the foregoing view will appear at all satisfactory to any one besides mj'- 

 self, — I believe, however, there is more in it tlian mere special pleading. 

 The case, undoubted!}-, is very perplexing ; but I have the confidence to 

 think, that the theory explains so well many facts, that I shall hold fast 

 by it, in the face of two or three puzzles, even as good ones as your third 

 objection. Believe me, my Dear Sir, yours very truly, 



Charles Darwin. 



Description of an improved Tilting Apparatus for emptying 

 Waggons at the termini of Railways, Shipping-Places, ^-c, as 

 used at the Magheramorne Lime-Works, Ireland. With a 

 Plate. By James Thomson, Esq., F.R.S.E., M.R.I.A., 

 F.R.S.S.A., Civil Engineer, Glasgow. Communicated by 

 the Royal Scottish Society of Arts.* 



The apparatus may be generally described as consisting of 

 three parts, viz : — 



1*/, The cast-iron brackets or quadrants for supporting the 



machine, Plate I. a a a. 

 2d, The tilting-frame upon which the waggon is placed, 



b b, — and 

 3rf, The malleable iron-swings for suspending the frame to 



the brackets, c c. 

 The supporting brackets a a a, are bolted to the wooden 

 frame d d, of a moveable shipping platform, by means of which 



* Read before the Royal Scottisli Society of Arts, and working model ex- 

 hibited, 10th January 1842, and the Soriety's Honorary Silver Medal award- 

 ed, 14th Noveiubcr 1842. 



