THE GEOLOGY OF HIYERAL SPEIYGS. 
207 
appears. Wells formed on tliis principle are called Artesian^ 
from tlieir L-aying* been supposed^ at one time_, to have been 
first made in tlie province of Artois^ in France ; bnt^ like 
many other modern inventions^ really of very high 
antiquity^ having been known thousands of years ago both to 
the Chinese and Egyptians. At the present da.y the country 
immediately around Thebes contains the remains of numerous 
Artesian borings which were probably made to supply that 
city with water. As an Artesian well is only an artificial spring, 
a brief account of the conditions which are favourable for 
making one will not be out of place here, especially as they 
are exactly those which govern the formation of natural springs. 
In making such a well the first step taken is to excavate an 
ordinary well to a sufficient depth to serve as a reservoir, in 
which the water of the Artesian spring may accumulate. From 
the bottom of this a bore is driven by means of an auger 
adapted to the puiqoose, worked by horse or steam power, and 
which is carried down until water is reached. AlthouQ^h at 
first the progress made in boring is pretty rapid, when a con- 
siderable depth is attained it becomes very slow in consequence 
of the frequent necessity of drawing up the boring rods to 
cleanse the borer of the debris wffiich accumulates in it, and 
of the great caution which is requisite to avoid breaking it. 
At a depth of about 1000 feet the progress made does not ex- 
ceed three feet a week under the most favourable circumstances. 
As water exists at considerable depths under great pressure, 
it follows that when reached by the bore it sometimes rises 
up it with much force ; indeed, it is upon this great 
subterranean pressure that the possibility of forming an 
Artesian Avell at all depends. The cost of making such a 
well is always great, and sometimes enormous. Upwards of 
£19,000 was expended upon one that was attempted some 
years ago with the view of providing the town of Southampton 
with water. The experience which was gained by this and, 
some other cases of costly failure has attracted more attention 
to the conditions which are necessary for the success of such 
an undertaking than had been previously given to them, and 
the subject is now pretty well understood. These conditions 
are (1) the existence of a porous stratum having a sufficient 
outcrop on the surface to collect an adequate amount of rain- 
fall, and passing down between two impermeable strata ; (2) 
the level of the outcropping portion of the porous stratum must 
be above that of the orifice of the well, so as to give a sufficient 
rise to the water; (3) there must be no outlet in the porous 
stratum by which its drainage can leak out, either in the shape 
of a dislocation, by which it can pass into lower strata, or a 
natural vent, by which it can rise to the surface at a lower 
