ON THE VOLCANIC PHENOMENA OF VESUVIUS. 407 
Practical results.—The sewer, on quitting the chocolate-coloured poz- 
zolana and lapilli of Bagnoli near the Stabil’ Patamia, will pass through 
the boulder and pebble deposit of unimportant thickness, after which the 
compact yellow tuff will be traversed for some considerable distance. 
Thence it will again pass into the boulder and pebble formation and 
gradually from coarser to finer pozzolana and sand of different kinds. 
For the above-mentioned geological regions it is impossible to deter- 
mine with precision the internal limits of these various formations, even 
at a small distance from the road. 
Even at the surface, the cliff section from La Pietra to Pozzuoli is 
one of the most intricate of the Campi Flegrzi, itself a most complicated 
region. 
~The canal by which the main mass of trachyte of Monte Olibano issued 
is probably situated beneath the highest eminence of the lava, that is, at 
Cariati (see fig. 3). If such is the case, and if the column of cooled 
lava is not of very great dimensions, the tunnel as projected would pass 
a little to the south; but if my calculations be erroneous or if there be 
any marked irregularities, it may be requisite to perforate the compact 
trachyte for some indeterminable distance. 
I do not believe that the sewer will meet with the scoriw beds which 
were found near the road in the Cumana Railway tunnel, after passing 
farther into the hill where, from the direction of the dip, these strata 
would be found at a higher level. But even here an exception might 
occur in the case that the inner declivity of the old filled-up crater of 
Monte Olibano extended far seawards. At any rate, it is to be hoped that 
this scoriz deposit will not be met with, because, with its structure full of 
interstices between irregular-shaped fragments, the exit of hot and 
poisonous vapour would occur with great ease. 
2. Temperature of the ground to be traversed——An atmosphere, the 
temperature of which exceeds the normal blood heat of man, is a condi- 
tion of the greatest importance in the employment of human labour. 
This normal temperature of the body can only be maintained by the 
evaporation of the perspiration, and the evaporation is in inverse proportion 
to the amount of aqueous saturation of the atmosphere. Man is capable 
of enduring for a relatively long time, in a dry atmosphere, a tempera- 
ture equal to that of boiling water; butif the air be saturated with 
moisture, the individual would be immediately subject to grave injury. 
It is therefore necessary not only to examine the temperature but also 
that of the relative saturation. 
Studying the ground from this point of view, the first fact that pre- 
sents itself is that at the Bocca Grande of the Solfatara, a very abundant 
escape of gas composed chiefly of water-vapour takes place, and presents 
at the point of its exit a temperature of 156° C. (December 12, 1889) 
which were it pure water-vapour would correspond to a pressure of 54 
atmospheres. We cannot determine at what rate the temperature and 
pressure of the vapour diminish as we recede from this point of maximum 
at the Bocca Grande, nor do we know in what proportion the temperature 
of the ground diminishes as we recede from the centre in a horizontal 
plain. If we assume empirically a diminution of pressure of gas and 
temperature of the ground proportional to the distance between two 
points at: which the temperature is known, we may get an idea of that of 
the tunnel where it approaches nearest to the Solfatara. 
In the Cumana Railway tunnel, under Monte Olibano, the working 
