Sidor som bilder
PDF
ePub

at one time fresh, and how it might become salter than the sea, even when its waters have not diminished.

H. I was going to say just now, I wondered that the rivers did not fill up the lakes to the brim, when none of the water could run away. But I now understand that the air can suck up as much or more than the rivers bring in; and when the air does suck up much more, the lake will get smaller and smaller till there is no water left.

U. O. Not exactly so, Henry. We need only suppose that the water will become less and less until the time when the lake receives the same quantity from its rivers as is sucked up by the air, at which point it may remain. To account for the fact that the Caspian Sea, for instance, was not filled up by the rivers that run into it, ancient writers and many moderns said that there must be an opening under ground through which the waters of the Caspian Sea ran into the Black Sea.

H. Is not that very likely, Sir ?

U. O. The thing itself would not be impossible ; but we know very well now that there cannot be any such connexion between the two seas.

Jane. That must be hard to find out. Did men dive down into the water to see if there was a hole ?

U. O. That, certainly, would be a pretty hard way of finding it out! The fact is, that it has been found that the waters of the Caspian Sea are 348 feet lower than those of the Black Sea. So if there was an opening, the waters of the Caspian would not run into the Black Sea, but those of the Black Sea into the Caspian, until both became of the same level.

H. I do not, after all, quite understand how the lakes can get low at all, Sir ; because all the water that is taken up into the air comes down again in rain, besides the great quantity which is brought in by rivers.

Frank. But all the rain does not fall upon the water: some falls upon the dry land.

H. Yes; but what falls upon the dry land runs into the water at last.

U. O. As a general rule, yes. But not into the particular water from which it was taken. Much of the air that has imbibed the water is carried away to distant regions. Besides, in such a country as Persia, the lakes receive back less than even the usual quantity. The country is very mountainous; and much of the rain that falls very near particular bodies of water, descends upon the more distant slope of the neighbouring mountains, and runs off into other

lakes or seas. Much of the rain also falls on dry and sandy deserts, from whence it never runs off, but sinks into the ground immediately. Indeed many of the rivers of Persia, as well as of Africa, are in this way completely lost in the deserts; that is to say, they never come to any lake or sea ; but losing their water, by little and little, as they pass through the deserts, they at last entirely waste away. · Mr. Dillon. We are all much obliged to you, Mr. Oldcastle, for your account of salt lakes : but although you suppose that the waters of certain lakes may have been originally salt, you do not say how that could happen.

U. O. Pray say it yourself, Mr. Dillon, while I take a glass of lemonade. Talking of deserts makes me feel thirsty.

Mr. D. I shall not be long in saying that. I can find no other account which satisfies me so well as that which supposes that the spots where the salt lakes are, were once covered by the sea. Then, when the sea retired, the hollows which now contain the lakes must have been left brim full of salt water, but their level began immediately to be lowered by evaporation, in the manner which you, Sir (to Uncle Oliver), have described ; and thus they go on becoming lower

H 2

and lower, and salter and salter, until they are brought down to that point at which the rivers bring in as much water as the evaporation takes. I prefer this account to any other; although there is another which does not materially differ from it, and to which I have no objection. According to this, such lakes were first rendered salt by salt substances which have been brought from the land by the rivers, and they gradually became as salt or salter than the sea, in precisely the same way as that I have just mentioned. In my opinion, both accounts may be true in different places; though, if we were obliged to make one account do for all salt lakes, I think the first would do best. We thus see how the lakes may become salter than the ocean. How much more salt than the sea, Mr. Oldcastle, is that lake Shahee in which you tell us that no fish can live ?

U. O. Fully one-third.

Mr. D. Some authors consider that when the sea retired from the country of the Caspian, it did not leave the several lakes as we now find them, but that the Black Sea, the Caspian Sea, and the Sea of Aral formed, with the plains and deserts which are now between them, and which border on them in the north and east, one great lake, or inland sea, as large as the Mediterranean, If we suppose this account to be correct, this grand lake must have been reduced to its present state by some circumstance which enabled it to discharge a portion of its waters, leaving dry much land which had been formerly covered. There are some who consider that this happened when the opening was made in the strait near Constantinople, which we now find between the Mediterranean and the Black Sea. But I have very much doubt on this point.

U. O. So have I, as to that ; but I am inclined to agree in the opinion, that in those parts much more of the land was formerly covered with water than at present. The districts you have mentioned afford only a sandy and salt soil, in which one may find an abundance of shells which resemble those of the Caspian Sea, and are different from those that are to be met with in the rivers that do now or did formerly run in those parts. This great desert region is also flat throughout, and contains a great number of salt marshes and lakes. A large part of it is composed of fine soft sand, in which one sinks to the ankle at every step, and on which I have many a time lain down on my cloak and slept through most of the night, either because the path could not be distinguished, or because the carriages stuck fast in the sand.

« FöregåendeFortsätt »