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Pliny the Younger (A.D. 62?–c.A.D. 113). Letters.
The Harvard Classics. 1909–14.

XLVIII. To Licinius Sura

I HAVE brought you as a little present out of 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, artificially formed for that purpose, from whence, after being detained a short time, it falls into the Larian lake. The nature of this spring is extremely curious; it ebbs and flows regularly three times a day. The increase and decrease are plainly visible, and exceedingly interesting to observe. You sit down by the side of the fountain, and while you are taking a repast and drinking its water, which is extremely cool, you see it gradually rise and fall. If you place a ring, or anything else, at the bottom, when it is dry, the water creeps gradually up, first gently washing, finally covering it entirely, and then little by little subsides again. If you wait long enough, you may see it thus alternately advance and recede three successive times. Shall we say that some secret current of air stops and opens the fountainhead, first rushing in and checking the flow and then, driven back by the counter-resistance of the water, escaping again; as we see in bottles, and other vessels of that nature, where, there not being a free and open passage, though you turn their necks perpendicularly or obliquely downwards, yet, the outward air obstructing the vent, they discharge their contents, as it were, by starts? Or may not this small collection of water be successively contracted and enlarged upon the same principle as the ebb and flow of the sea? Or, again, as those rivers which discharge themselves into the sea, meeting with contrary winds and the swell of the ocean, are forced back in their channels, so, in the same way, 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, and while it is recruiting its discharges, the stream in consequence flows more slowly and in less quantity, but, when it has collected its due measure, runs on again in its usual strength and fulness? Or, lastly, is there I know not what kind of subterranean counterpoise, that throws up the water when the fountain is dry, and keeps it back when it is full? You, who are so well qualified for the enquiry, will examine into the causes of this wonderful phenomenon; it will be sufficient for me if I have given you an adequate description of it. Farewell.