490 PLINY'S NATURAL 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 31 of the earth, to see whether, beginning 

 witb 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 33 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 33 sand, fine sea 34 - sand, and charcoal 35 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 36 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 "Com." 32 "Sabulum." 



33 " Sabulum masculum." Coarse, reddish sand, Dalechamps says. 



34 "Arena." 35 See B. xvii. c. 3. 



36 An inconvenience neutralized in a considerable degree by Davy's in- 

 vention of the safety-lamp. 



