334 Mr Forbes's Physical Notices of the Bay of Naples. 



tracted from a letter received from Italy shortly after the 

 event, and in no long time the account of a phenomenon ar- 

 rived with which it had doubtless a connection, the eruption 

 of Vesuvius on the 21st of March the same year. The loca- 

 lity of the principal shock, it is interesting to observe, coin- 

 cides nearly with the ancient point of emission of the Ischian 

 volcano, and from the direction in which it reached Casamic- 

 ciola, it may very possibly have originated in the actual Campo 

 del Arso. It is interesting to observe, that this earthquake 

 was the precursor of this eruption, and that instead of being, 

 as Mr Scrope in his paper in the Geological Transactions 

 suspected, the intimation of the direction of volcanic ener- 

 gy to this its ancient seat, it thus proved to be merely the 

 concomitant of its paroxysm in its established point of emis- 

 sion. Thus having briefly noticed the more conspicuous traits 

 of the physical history of Ischia, we proceed directly to give 

 some account of its constitution and products. 



The great mass of the island is composed of a rather friable 



of the barometer widi the occurrence of earthquakes is by La Cotte in 

 the Journal de Physique, vol. G5. Many of the shocks were extreme- 

 ly slight and at a great distance from the place of observation. But I 

 shall select at random the more conspicuous which occurred during the 

 space of only four years ; I shall classify the state of the barometer under 

 the heads of 



Great Elevation. Great Depression. Great Variation. Stationary. 



24th Jan. 1775, 6th and 22d Oct. 1775, 4th Feb. 1775, 20th June 1775, 



5th Aug. 1776, 7th Feb. 1776, 14th Feb. 1775, 8th Sept. 1775, 



5th May 1778, 1st Oct. 1777, 27th Feb. 1776, 30th Dec. 1775, 



18th Jan. 1778, 4th July 1777, 30th Jan. 1776, 



18th Feb. 1778, 23d Sept. 1777, 22d Apr. 1776, 



2d Apr. 1778, 9th June 1776, 



20th Apr. 1778, 1st July 1776, 



1st Oct. 1778, 4th Aug. 1776, 



31st Dec. 1778, 6th Sept. 1776, 



6th June 1777, 



15th Oct. 1777, 



31st July 1778, 



19-26th Dec 1778- 



Here the ratio of the times when the barometer was aifected to when it 

 was not are as 1 7 : 13, and there was only one instance of great elevation to 

 three of great depression. But had the observations included a longer 

 period, and only the more notable earthquakes, both these ratios would 

 probably have been much higher. 



