236 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 24, 



rate of change of level, differ considerably from those of Cavalier Ni- 

 collini, who has made a series of observations on the depth of the water 

 between the years 1822 and 1838. They prove the important fact, 

 that a gradual change of level is taking place ; but according to his 

 calculations it is only at the rate of seven millimetres, or less than 

 one-third of an iach annually. 



Upon examining his tables, however, I find that though the entries 

 repeatedly descend to zero on the scale of his hydrometer, they never 

 go below it ; and on the other hand, he rejects all the high num- 

 bers, confining his data to the three lowest of each year, from which 

 he infers that the amount of rise in these sixteen years was only 112 

 millimetres, or about 4^ inches, which, divided by sixteen, the total 

 number of years of observation, gives the above-mentioned result. 

 It is obvious that the mean rate which he arrives at by this mode of 

 calculation must be too small ; this however does not diminish the 

 value of his observations, and I regret I cannot avail myself of them 

 in the present comparison, because I am unable to discover the rela- 

 tive height of his low-water mark with the bases of the columns, or 

 even with the level of the floor. 



In consequence of an accident to his hydrometer, he has erected 

 a new one which gives the depth at high water above the floor (sopra 

 del piano). This, in January 1838, was half a metre, or about 20 

 inches, being about 8 inches below my observation made seven years 

 and a half afterwards, agreeing very nearly with the rate deduced 

 from the observations made by Professor Forbes, Mr. Babbage and 

 myself. 



It appears to me that this depression has been going on for many 

 years, probably since 1538, the date of the last paroxysmal elevation, 

 according to the contemporary accounts published by Sir Wilham 

 Hamilton, and mentioned by Mr. Lyell in his account of the building. 



In *La Vera Antichita di Pozzuoli,' printed at Rome, 1652, there 

 is a bird's-eye view of this locality, in which the three columns are re- 

 presented standing in the garden of a villa at a considerable distance 

 from the sea, and between it and the building are seen two churches, 

 Santa Maria Gratiarum and Jesu Maria ; and in the * Guida di Poz- 

 zuoli,' 1 709, the columns are thus noticed : " Nell giardino oggi di 

 Alessandra Flauto si vedono tre colonne maravigliose tutti di un 

 pezzo." The whole of the intervening space with its buildings has 

 disappeared, and there are two sea-walls standing in the sea parallel 

 to the present one, built to protect the road ; one of them is, if I re- 

 collect right, 20 or 30 feet from the shore, and the other about 

 double the distance. 



The church of Madonna del Assunto is now surrounded by the sea 

 and connected with the land by a causeway, which it has been found 

 necessary to raise, and the surface of the water is level with the floor 

 of the building. 



The road from Naples along shore was being raised at the time I 

 visited it, and from every thing I could learn upon the spot from the 

 old people, a gradual subsidence has been going on for many years. 



The following appears to be the history of the changes in the rela- 



