1824.] Geological Society. 65 



opinion he was supported by the testimony of Dr. Maskelyne, 

 Bernoulli, and many eminent astronomers who were quoted. 

 Notwithstanding its high recommendations, this method has not 

 been successfully adopted in practice, and has even led to some 

 awkward anomalies, on account of its having been customary 

 to take the moon's centre reduced to the meridian, and to com- 

 pare it with the apparent places of stars passing the meridian 

 about the same time in auy parallel of declination. 



The newly proposed method consists in merely observing with 

 a transit instrument, the differences of right ascension between 

 the border of the moon, and certain fixed stars previously 

 agreed upon, restricting the observations to such stars as differ 

 very tittle in declination from the moon, and denominated moon 

 culminating stars. The attention of astronomers has been called 

 to this method by M. JSTicolai, of Manheim, in several numbers 

 of Schumacher's Nachrichteu. It is quite independent of the 

 errors of the Lunar Tables (except so far as the moon's horary 

 motion in AR is concerned). It does not involve the quantity 

 of the earth's compression. It does not require a correct 

 knowledge of the position of the star observed, nor does an 

 error of a few seconds in the clock sensibly affect the result. 

 Hence much trouble is avoided, many causes of error pre- 

 cluded ; besides all which, the method is universal. 



GEOLOGICAL SOCIETY. 



May 21. — The reading of the paper " On the Geology of the 

 Ponza Islands in the Mediterranean ; " by George Poulett 

 Scrope, Esq. MGS. was concluded. 



The Ponza Islands lie off the coast of Italy, opposite Terra- 

 cina and Gaieta. They consist of Ponza (anciently Pandataria), 

 Palmarola, and some islets ; Ventotiene and San Stefano con- 

 nect them with Ischia. The harbour of Ponza is excellent. 

 Dolomieu's Memoire sur les Isles Ponces excited curiosity, but 

 is too general to satisfy it. These islands are composed of 

 rocks, of the Trachytic series, and presenting fine sections 

 along their coasts, enabled the author to clear up many doubts 

 and errors which the mere investigations of inland localities 

 have caused to be affixed to this formation. 



The Isle of Ponza is long and very narrow, and is eroded by 

 the sea into deep concavities. Harder masses left along its 

 shores show that it once was broader, and protruding ledges 

 mark its former connexion with Quannone and La Gabbi;t. 

 Prismatic trachyte, variously coloured and disposed, forms the 

 ossature of the island. It is constantly accompanied by, and 

 alternates with, a semi-vitreous trachytic conglomerate, formed 

 of minute pulverulent matter enclosing fragments of trachyte. 

 The prismatic trachyte seems to have been forcibly injected 

 through the conglomerate, and wherever it touches the latter 



Ne^o Series, vol via. f 



