25$ On the Length of the Seconds Pendulum. 



To conclude. Old red sandstone is a term become so truly 

 ridiculous, from the perfectly contrary application of it, which 

 equally confident Geognosts have made in England, that the 

 sooner it is expunged from British phraseologv, the better ; nor 

 can I think there is much sense, in substituting vew in the place 

 of " vale of Eden red sandstone," which latter no one could well 

 mistake. It is not a little curious, that the site of this uncon- 

 formable rock around Carlisle, is said, in the recently published 

 2d Edition of A Course of Lectures on Geology, pretending to 

 give a copy and account of Smith's Map, to be occupied by 

 Graywakke ! : and similar assertions, are even more expressly 

 made, regarding the vicinity of Liverpool, northward ! — When, 

 alas !, will it become the general practice in Geology, to read 

 and observe first, and ivrite or Lecture afterwards !. 

 I am, 



Your obedient servant, 



A Constant Reader. 



P. S. The occurrence of Magnesian Limestone beds, in the 

 vale of Eden strata, was very distinctly inferred, in p. 173 of your 

 xlvth volume. 



LXIV. Oil the Length of the Seco}id^ Pendulum, — on the re- 

 ciprocal Action of Pevduliims, — and on the Swiftness of 

 Sou7id in differevt Substances, — by J/. Lai'lace ; — being the 

 Substance of several Paper's successively read to the French 

 Academy of Sciences on the 2Sth of October, 25th of Novem- 

 ber y and 2'dd of December 1816. 



Length of the Seconds FenduUim. 



\ HE variation of gravity is the most proper phsenomenon to 

 enlighten us concerning the constitution of the earth. The 

 causes upon which it depends are not limited to ihe terrestrial 

 surface, but extend to the deepest beds or strata ; so that an in- 

 considerable irregularity in a stratum situated a thousand leagues 

 from the surface has a sensible effect on the length of the se- 

 conds pendulum. It may be concluded that the deeper this ir- 

 regularity is, th? more extended will be its effect on the earth : 

 so that we may thus estimate that depth by the degree of cor- 

 responding irregularity in the length of the pendulum. It is very 

 important, therefore, to give to the observations of this length 

 such a precision as may assure us, that the anomalies observed 

 are not owing to error. Already a great number of experiments 

 on this subject have been made in both hemispheres ; and al- 

 though they leave much to desire, yet their regular progress \i\ 



conformity 



