340 Archdeacon Pratt on the Curvature of the Indian Arc. 



of the continual examination, and correction when required, 

 of the tables of the deviation with the ship's head on different 

 points. The inverse proceeding, or the endeavour to correct 

 maps formed on a sufficiently extended basis of cooi'dinated land 

 and sea observations by observations at sea which have not been 

 corrected for deviation, is hardly to be recommended : and doubt- 

 less as variation maps become more deserving in themselves, and 

 their value is better known, will be less likely to be practised. 



XLV. On the Curvature of the Indian Arc ; and the great Geo- 

 logical Law, that the various parts of the Solid Crust of the 

 Earth are perpetually undergoing a Change of Level. By 

 Archdeacon Pratt. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



OF the existence of the great law brought to light by the 

 science of geology, that the various parts of the earth's 

 surface are perpetually undergoing a change of level, there can 

 be now no reasonable doubt. The observations made on land- 

 marks on the coast of the Baltic, — the singular phsenomenon of 

 the columns of the temple of Serapis near Naples, although now 

 above the water-level, as they must have been when the temple 

 was built, and yet showing marks of having been below the sur- 

 face, in the perforations of a kind of sea shell-fish at the present 

 day flourishing in the neighbouring waters, — and the original 

 researches and bold generalizations of Mr. Darwin among the 

 ancient and recent coral formations of the South Seas, — combine 

 to establish the law in the most satisfactory manner. 



2. To assign a cause of this law may be more difficult than to 

 recognize its existence. Mr. Babbage, and others after him, 

 have attributed it to the expansion and contraction of masses 

 of rock beneath, cavised by accessions and withdrawals of heat 

 through chemical agencies. Obscure as the origin of the law 

 may be, the importance of its discovery cannot be over-estimated, 

 as it supplies so clear an explanation of the wonderful alterna- 

 tions of level which miist have taken place to produce the phse- 

 nomena of fresh- and salt-water formations which geology brings 

 to light. 



3. Satisfactorily as a law of nature may have been made out, 

 it can never be out of place to bring forward new illustrations, 

 especially in one of such importance as this law possesses. I 

 shall therefore make no apology for asking you to admit into 

 your Magazine a proof drawn from an entirely new province, 

 that of mathematical physics. The circumstances and arguments 

 on which it is based I will now relate. 



