266 PHYSICAL GEOGRAPHY. 



or mass of rock impervious to water, as A A ; B B a stratum through which it readily 

 circulates, and which crops out or rises from beneath A A on each side ; c c also a rock 

 through which it cannot pass. It is clear that rain, falling on the permeable stratum 

 where it crops out, and being absorbed by it, yet prevented from passing downward 

 under the action of gravity by the rock c c, will travel laterally through the stratum 



under the town, unable however to find 

 its level, and force its way to the surface 

 through the superincumbent rock A A. 



Artesian Well. This is a condition in which an artificial 



boring at w, through A A to B B, will liberate 



the water of the latter, which will rise in the vent to the surface of the plain, in proportion 

 as its source is elevated above it. Artesian wells, notwithstanding their modern name 

 and its local derivation, appear to have been well known in various countries and in 

 ancient times, without perhaps any apprehension of their principle. Niebuhr quotes an 

 ancient writer as saying, " Wells are sunk in the oases from two to four hundred yards 

 in depth (the yard in question being equal to half a foot), whence water rises and flows 

 over." 



3. Reciprocating. There are springs which exhibit phenomena analogous to the flux 

 and reflux of the tides of the ocean, some at regular intervals during the day, and others 

 at more distant and uncertain periods. In one of the two letters addressed by the 

 younger Pliny to Licinius, he describes a spring of this kind by the Larian lake the 

 modern Lake of Como : "I have brought you," he remarked, " as a present, out o'f the 

 country, a query which well deserves the consideration of your extensive knowledge. 

 There is a spring which rises in a neighbouring mountain, and, running among the rocks> 

 is received into a little banqueting room, from whence, after the force of its current is a 

 little restrained, it falls into the Larian lake. The nature of this spring is extremely 

 surprising ; it ebbs and flows regularly three times a day. The increase and decrease is 

 plainly visible and very amusing to observers. You sit down by the side of the fountain ; 

 and whilst you are taking a repast, and drinking its water, which is extremely cool, you 

 see it gradually rise and fall. If you place a rig or any thing else at the bottom when 

 it is dry, the stream reaches it by degrees till it is entirely covered, and then gently 

 retires ; and if you wait you may see it thus alternately advance and recede three succes- 

 sive times. Shall we say that some secret current of air stops and opens the fountain- 

 head as it approaches to or retires from it, as we see in bottles, and other vessels of that 

 nature, when there is not a free and open passage ; though you turn their necks down- 

 wards, yet, the outward air obstructing the vent, they discharge their contents as it were 

 by starts ? But may it not be accounted for upon the same principle as the flux and 

 reflux of the sea? Or as those rivers which discharge themselves into the sea, meeting 

 with contrary winds and the swell of the ocean, are forced back into their channels, so 

 may there not be something that checks this fountain, for a time, in its progress ? Or is 

 there rather a certain reservoir that contains these waters in the bowels of the earth, 

 which, while it is recruiting its discharges, the stream flows more slowly and in less quan- 

 tity ; but, when it has collected its due measure, it runs again in its usual strength and 

 fulness ? Or, lastly, is there I know not what kind of subterraneous counterpoise, that 

 throws up the water when the fountain is dry, and stops it when it is full ? You, who 

 are so well qualified for the enquiry, will examine the reasons of this wonderful pheno- 

 menon : it will be sufficient for me, if I have given you a clear description of it. Fare- 

 well." 



The fact of the flow and ebb was reported, in antiquity, of a fountain, the celebrity of 

 which is co-extensive with the prevalence of Christianity itself : 



