490 
pliisty's NATUEAL HISTOET. [Book XXXI. 
water, by presenting white spots, or an uniformly green appear- 
ance : for where the stratum is black the springs are mostly 
not of a permanent nature. The presence of potter's clay 
always puts an end to all hopes of finding water, and the ex- 
cavation is immediately abandoned ; an eye being carefully 
kept to the strata^^ of the earth, to see whether, beginning 
with black mould, it successively presents the appearances 
above-mentioned. The water is always fresh that is found 
in argillaceous soils, but in a stratum of tufa it is colder than 
elsewhere ; this, indeed, being a soil which is highly approved 
of, as having a tendency to make the water pure and extremely 
light to the stomach, and, by its action as a filter, to withhold 
all impurities. The presence of sand^^ gives indications of 
springs of but limited extent, and of water impregnated with 
slime ; while that of gravel announces the presence of water of 
excellent flavour, but not to be depended upon for permanence. 
Male^^ sand, fine sea^^-sand, and charcoaP^ earth, yield a con- 
stant supply of water of a highly wholesome quality ; but it 
is the presence of red stones that is the most to be depended 
upon, and the water found there is of the very finest quality. 
Craggy localities at the foot of mountains, and silicious soils, 
are equally good; in addition to which, the water found there 
is cooler than elsewhere. 
In boring for water, the soil should always become more 
and more humid, and, the deeper the descent, with the greater 
facility the implements should penetrate. In deep-sunk wells, 
the presence of sulphureous^^ or aluminous substances is fatal 
to the sinkers ; a danger that may be guarded against by letting 
down a lighted lamp, and ascertaining whether the flame is 
extinguished. "When such is found to be the case, it is the 
practice to sink vent-holes on each side of the well, both right 
and left, in order to receive and carry off the noxious exhala- 
tions. Independently of these evils, the air becomes heavier, 
from the great depth merely of the excavation, an inconvenience 
which is remedied by keeping up a continual circulation with 
ventilators of linen cloth. As soon as water is reached, walls 
31 Coria." 33 « Sabulum." 
33 " Sabulum masculum." Coarse, reddish sand, Dalechamps says. 
31 "Arena." 35 gee B. xvii. c. 3. 
36 An inconvenience neutralized in a considerable degree by Davy's in- 
vention of the safety-lamp. 
