Imágenes de páginas
PDF
EPUB

touch, as, by having its fire drawn into the mixture, it is become more capable of drawing and receiving it from the hand; and, through the basin, the water loses its fire that kept it fluid; so it becomes ice. One would expect that, from all this attracted acquisition of fire to the composition, it should become warmer; and, in fact, the snow and salt dissolve at the same time into water, without freezing.

B. FRANKLIN.

Peter Franklin, Newport, Rhode Island.

ON THE SALTNESS OF SEAWATER.

London, May 7, 1760. * * It has, indeed, as you observe, been the opinion of some very great naturalists, that the sea is salt only from the dissolution of mineral or rocksalt which its waters happen to meet with. But this opinion takes it for granted that all water was originally fresh, of which we can have no proof. I own I am inclined to a different opinion, and rather think all the water on this globe was originally salt, and that the fresh water we find in springs and rivers is the produce of distillation. The sun raises the vapours from the sea, which form clouds, and fall in rain upon the land, and springs and rivers are formed of that rain. As to the rock-salt found in mines, I conceive that, instead of communicating its saltness to the sea, it is itself drawn from the sea, and that, of course, the sea is now fresher than it was originally. This is only another effect of na. ture's distillery, and might be performed various ways.

It is evident, from the quantities of seashells, and the bones and teeth of fishes found in high lands, that the sea has formerly covered them. Then either the sea has been higher than it now is, and

has fallen away from those high lands, or they have been lower than they are, and were lifted up out of the water to their present height by some internal mighty force, such as we still feel some remains of when whole continents are moved by earthquakes. In either case it may be supposed that large hollows, or valleys among hills, might be left filled with seawater, which, evaporating, and the Auid part dry. ing away in a course of years, would leave the salt covering the bottom; and that salt, coming afterward to be covered with earth from the neighbouring hills, could only be found by digging through that earth. Or, as we know from their effects that there are deep, fiery caverns under the earth, and even under the sea, if at any time the sea leaks into any of them, the fluid parts of the water must evaporate from that heat, and pass off through some volcano, while the salt remains, and, by degrees and continual accretion, becomes a great mass. Thus the cavern may at length be filled, and the volcano connected with it cease burning, as many, it is said, have done; and future miners, penetrating such cavern, find what we call a salt-mine. This is a fancy I had on visiting the salt-mines at Northwich with my son. I send you a piece of the rock-salt which he brought up with him out of the mine.

B. FRANKLIN.

To Miss Stephenson.
SALT WATER RENDERED FRESH BY DISTILLATION.
METHOD OF RELIEVING THIRST BY SEAWATER.

Craven-street, August 10, 1761. We are to set out this week for Holland, where we may possibly spend a month, but purpose to be at home again before the coronation. I could not go without taking leave of you by a line at least, when I am so many letters in your debt.

In yours of May 19, which I have before me, you speak of the ease with which salt water may be made fresh by distillation, supposing it to be, as I had said, that in evaporation the air would take up water, but not the salt that was mixed with it. It is true that distilled seawater will not be salt, but there are other disagreeable qualities that rise with the water, in distillation; which, indeed, several besides Dr. Hales have endeavoured by some means to prevent, but as yet their methods have not been brought much into use.

I have a singular opinion on this subject, which I will venture to communicate to you, though I doubt you will rank it among my whims. It is certain that the skin has imbibing as well as discharging pores; witness the effects of a blistering-plaster, &c. I have read that a man, hired by a physician to stand, by way of experiment, in the open air naked during a moist night, weighed near three pounds heavier in the morning. I have often observed myself, that however thirsty I may have been before going into the water to swim, I am never long so in the water. These imbibing pores, however, are very fine; perhaps fine enough, in filters ing, to separate salt from water; for though I have soaked (by swimming, when a boy) several hours in the day, for several days successively, in salt water, I never found my blood and juices salted by that means, so as to make me thirsty or feel a salt taste in my mouth; and it is remarkable that the flesh of seafish, though bred in water, is not salt. Hence I imagined that if people at sea, distressed by thirst, when their fresh water is unfortunately spent, would make bathing-tubs of their empty water-casks, and, filling them with seawater, sit in them an hour or two each day, they might be greatly relieved. Perhaps keeping their clothes, constantly wet might have an almost equal effect; and this without danger of catching cold. Men do

VOL. II.-23

not catch cold by wet clothes at sea. Damp, but not wet linen, may possibly give colds ; but no one catches cold by bathing, and no clothes can be wetter than water itself. Why damp clothes should then occasion colds, is a curious question, the discussion of which I reserve for a future letter or some future conversation.

Adieu, my little philosopher. Present my respectful compliments to the good ladies your aunts, and to Miss Pitt, and believe me ever

B. FRANKLIN.

To the same.

TENDENCY OF RIVERS TO THE SEA.-EFFECTS OF THE SUN'S RAYS ON CLOTHS OF DIFFERENT COLOURS.

September 20, 1761. MY DEAR FRIEND, It is, as you observed in our late conversation, a very general opinion, that all rivers run into the sea, or deposite their waters there. 'Tis a kind of audacity to call such general opinions in question, and may subject one to censure. But we must hazard something in what we think the cause of truth: and if we propose our objections modestly, we shall, though mistaken, deserve a censure less severe than when we are both mistaken and insolent.

That some rivers run into the sea is beyond a doubt : such, for instance, are the Amazons, and, I think, the Oronoko and the Mississippi. The proof is, that their waters are fresh quite to the sea, and out to some distance from the land. Our question is, whether the fresh waters of those rivers, whose beds are filled with salt water to a considerable distance up from the sea (as the Thames, the Delaware, and the rivers that communicate with Chesapeake Bay in Virginia), do ever arrive at the sea ? And as I

suspect they do not, I am now to acquaint you with my reasons; or, if they are not allowed to be reasons, my conceptions at least of this matter.

The common supply of rivers is from springs, which draw their origin from rain that has soaked into the earth. The union of a number of springs forms a river. The waters, as they run exposed to the sun, air, and wind, are continually evaporating. Hence, in travelling, one may often see where a river runs, by a long bluish mist over it, though we are at such a distance as not to see the river itself. The quantity of this evaporation is greater or less, in proportion to the surface exposed by the same quantity of water to those causes of evaporation. While the river runs in a narrow, confined channel in the upper hilly country, only a small surface is exposed; a greater as the river widens. Now if a river ends in a lake, as some do, whereby its waters are spread so wide as that the evaporation is equal to the sum of all its springs, that lake will never overflow; and if, instead of ending in a lake, it was drawn into greater length as a river, so as to expose à surface equal in the whole to that lake, the evaporation would be equal, and such river would end as a canal; when the ignorant might suppose, as they actually do in such cases, that the river loses itself by running under ground, whereas, in truth, it has run up into the air.

Now, how many rivers that are open to the sea widen much before they arrive at it, not merely by the additional waters they receive, but by having their course stopped by the opposing flood-tide ; by being turned back twice in twenty-four hours, and by finding broader beds in the low flat countries to dilate themselves in; hence the evaporation of the fresh water is proportionably increased, so that in some rivers it may equal the springs of supply. In such cases the salt water comes up the river, and meets the fresh in that part where, if there were a

« AnteriorContinuar »