« ZurückWeiter »
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 filtering, 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 salt 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
To the same.
TENDENCY OF RIVERS TO THE SEA.-EFFECTS OF THE SUN'S RAYS ON CLOTHS OF DIFFERENT COLOURS.
MY DEAR FRIEND,
September 20, 1761.
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 a 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
wall or bank of earth across, from side to side, the river would form a lake, fuller indeed at some times than at others, according to the seasons, but whose evaporation would, one time with another, be equal to its supply.
When the communication between the two kinds of water is open, this supposed wall of separation may be conceived as a moveable one, which is not only pushed some miles higher up the river by every flood-tide from the sea, and carried down again as far by every tide of ebb, but which has even this space of vibration removed nearer to the sea in wet seasons, when the springs and brooks in the upper country are augmented by the falling rains, so as to swell the river, and farther from the sea in dry seasons.
Within a few miles above and below this moveable line of separation, the different waters mix a little, partly by their motion to and fro, and partly from the greater gravity of the salt water, which inclines it to run under the fresh, while the fresh water, being lighter, runs over the salt.
Cast your eye on the map of North America, and observe the Bay of Chesapeake, in Virginia, mentioned above; you will see, communicating with it by their mouths, the great rivers Susquehanna, Potomac, Rappahannoc, York, and James, besides a number of smaller streams, each as big as the Thames. It has been proposed by philosophical writers, that to compute how much water any river discharges into the sea in a given time, we should measure its depth and swiftness at any part above the tide as for the Thames, at Kingston or Windsor. But can one imagine, that if all the water of those vast rivers went to the sea, it would not first have pushed the salt water out of that narrowmouthed bay, and filled it with fresh? The Susquehanna alone would seem to be sufficient for this,
if it were not for the loss by evaporation. And yet that bay is salt quite up to Annapolis.
As to our other subject, the different degrees of heat imbibed from the sun's rays by cloths of different colours, since I cannot find the notes of my experiment to send you, I must give it as well as I can from memory.
But first let me mention an experiment you may easily make yourself. Walk but a quarter of an hour in your garden when the sun shines, with a part of your dress white and a part black; then apply your hand to them alternately, and you will find a very great difference in their warmth. The black will be quite hot to the touch, the white still cool.
Another. Try to fire the paper with a burning glass. If it is white, you will not easily burn it; but if you bring the focus to a black spot, or upon letters written or printed, the paper will immediately be on fire under the letters.
Thus fullers and dyers find black cloths, of equal thickness with white ones, and hung out equally wet, dry in the sun much sooner than the white, being more readily heated by the sun's rays. It is the same before a fire, the heat of which sooner penetrates black stockings than white ones, and so is apt sooner to burn a man's shins. Also beer much sooner warms in a black mug set before the fire than in a white one, or a bright silver tankard.
My experiment was this. I took a number of little pieces of broadcloth from a tailor's pattern card, of various colours. There were black, deep blue, lighter blue, green, purple, red, yellow, white, and other colours or shades of colours. I laid them all out upon the snow in a bright sunshiny morning. In a few hours (I cannot now be exact as to the time) the black, being warmed most by the sun, was sunk so low as to be below the stroke of the sun's rays; the dark blue almost as low, the