176 Capt. Sabine's Reply to Mr. Henderson's Remarks. 



quantity of fresh-water limestone or travertine on the side of 

 Civita Vecchia. It is first seen constituting large platforms 

 near Mala Grotta and Guido ; it then ceases : but after having 

 passed the Pulidoro and the hamlet of the same name, consi- 

 derable masses of fresh-water limestone are traversed, forming 

 salient portions, and appearing to advance towards the sea in 

 the manner of a lava. It is thickest and most abundant at 

 Monterone, where it rests on a rock having all the chai'acters 

 of a transition formation. 



The celebrated cascades of Tivoli are not due to escarpments 

 of the compact limestone, forming the mass of these hills, but 

 to a stoppageof the valley produced by deposits from the waters 

 which flow from it, and which were much mox'e charged for- 

 merly than at present with carbonate of lime. The agitation of 

 the waters gives rise to undulations in this deposit, not observ- 

 able in the plain; and the less abundant i)recipitation allows 

 the limestone to acquire a texture and crystalline aspect, re- 

 moving it from travertine and rendering it more like alabaster. 

 The same facts, owing to the same causes, are observable at the 

 beautiful cascades of Terni. Compact fresh-water limestone 

 or travertine is first met with in the environs and lower parts ; 

 and afterwards at Rieti, at the confluence of the Velino and 

 the Nera, this little river precipitates itself over a bar of cry- 

 stalline concretionary limestone, formed in the same way and 

 on the same fundamental compact limestone as at Tivoli. 

 M. d'Halloy has observed fresh-water shells in the concretion- 

 ary limestone. H. T. D. B. 

 [To be continued.] 



XXXII. Reply to Mr. Henderson's Remarks on Captain 

 Sabine's Pendulum Observations. By Capt. E. Sabine, R.A. 

 F.R.S. Sfc. 

 To the Editors of the Philosophical Magazine and Annals. 



Gentlemen, 



IN reply to Mr. Henderson's communication in your last 

 Number, T beg to acquaint hiin that a detailed account of 

 the corrections of my pendulum experiments was presented to 

 tlie Royal Society the day after my return from the continent, 

 and read the same week, being in the commencement of last 

 June * ; and that I expect it will be printed at the close of 

 a paper containing a continuation of the same experiments 

 connecting Paris and London, in which I have been lately 

 engaged. 



Mr. Henderson has justly characterized the correction of the 



• See our last Number, p. 143. — Edit. 



length 



