ON THE VOLCANIC PHENOMENA OF VESUVIUS. 409 
fore the walling of the sewer would suffer from the action of the sulphuric 
acid which would be formed by the oxidation of the sulphur exhalations. 
This acid, attacking the rocks, would tend to convert the cuniculus into 
an alum cave. This effect would, however, be to a certain extent miti- 
gated in the lower part by the continuous flow of sewage relatively cold. 
In proof of this numerous old Roman walls are entirely broken up by 
such action in sites much farther removed from such an active fumarolic 
focus. 
4. Thermo-mineral waters.—The wells that one encounters in this region 
are mostly situated very near the sea, and offer few facts from which 
deductions of much value can be drawn. Their water possesses nearly 
always a temperature exceeding 50° C., and on account of the porosity of 
the soil they are capable of affording a constant supply when heavily 
pumped. In the crater of the Solfatara we have a well of thermo- 
mineral water, the level of the water in which is much above the sea- 
level. But this, however, probably depends on its being closed in a 
crater basin rendered impermeable by the deposits of clay due to the 
decomposition of the trachytic rocks at Pozzuoli. It is known that the 
subterranean drainage level rises rather rapidly as we recede from 
the shore, so that at the Montagna Spaccata it attains 13m. This 
occurs in formations in part identical and in part resembling those which 
the sewer must traverse in the region discussed in this paper. 
Practical results—It is not probable that the drainage level of the 
ground will be as high as that of the sewer. Nevertheless it will not be 
found much beneath the bottom of the tunnel, and where open-structured 
rocks are traversed the humidity of the atmosphere will be much 
augmented. 
d. Change of level_—The sewer must traverse the most classical region 
renowned for the clear and unmistakable evidence of the oscillation of 
level of many metres during the historic period; a phenomenon still in 
progress. 
It is not necessary to record here the studies of so many scientists 
who have demonstrated that the coast of Pozzuoli and other parts of the 
gulfs of Naples and Gaeta are sinking at a rate of 7 to 14mm. every 
year. In the last dozen years that I have studied this region, I have 
been able to collect such numerous and important facts as to remove 
any doubt that the change of level is nearer to 13 to 14mm. than a 
minimum of 7 mm. 
Many new facts in confirmation of this form of bradyseismic move- 
ments observed by me in the last years will be found in the reports 
of this Committee from 1884 to 1889. Unfortunately no accurate 
observations to determine the rate of this present lowering have been 
made, which would have been ‘of great use in many engineering and 
building questions. Also, it is not only necessary to know whether there 
he lowering of the ground and at what rate, but also the main interest 
in the present case would be to know if the progress is uniform each 
season or each year, and above all if it is uniform for all the Naples region. 
Individually, I am inclined to think that change of level has been in 
great part the cause of the abandonment of Pestum and other analogously 
situated localities, as well as of the augmentation of malaria during the 
Middle Ages in the neighbourhood of Ostia and the Roman marshes near 
the sea, and also of the diminution of the navigability of the Tiber. 
The province and the city of Naples should establish several instru- 
