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tion, 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 sea-water will not be salt, but there are other disagreeable qualities that rise with the water in distillation ; which indeed several besides Dr. Hales have endeavored 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 sea-fish, though bred in salt water, is not salt. Hence I imagine, 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 sea-water, 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 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.
[To Miss STEvenson.]
TENDENCY OF RIVERS TO THE SEA — EFFECTS OF THE SUN'S RAYS ON CLOTHS OF DIFFERENT COLORS.
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 deposit their waters there. 'T is 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 Amazon, and I think the Oronoco 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 movable 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 further from the sea in dry seasons. Within a few miles above and below this movable line of separation, the different waters mix a little, partly by their motion to and fro, and partly from the greater specific 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, Rappahannock, 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 narrow-mouthed bay, and filled it with fresh 2 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 colors, 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 in a bright silver tankard. My experiment was this. I took a number of little square pieces of broadcloth from a tailor's pattern-card, of various colors. There were black, deep blue, lighter blue, green, purple, red, yellow, white, and other colors or shades of colors. 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 lighter blue not quite so much as the dark, the other colors less as they were lighter; and the quite white remained on the surface of the snow, not having entered it at all. What signifies philosophy that does not apply to some use ? May we not learn from hence that black clothes are not so fit to wear in a hot, sunny climate or season as white ones, because in such clothes the body is more heated by the sun when we walk abroad, and are at the same time heated by the exercise, which double heat is apt to bring on putrid, dangerous fevers ? That soldiers and seamen, who must march and labor in the sun, should in the East or West Indies have an uniform of white 2 That summer hats, for men or women, should be white, as repelling that heat which gives headaches to many, and to some the fatal stroke that the French call the coup de soleil 2 That the ladies' summer hats, however, should be lined with black, as not reverberating on their faces those rays which are reflected upwards from the earth or water ? That the putting a white cap of paper or linen within the crown of a black hat, as some do, will not keep out the heat, though it would if placed without 2 That fruit-walls, being blacked, may receive so much heat from the sun in the day-time as to continue warm in some degree through the night, and thereby preserve the fruit from frosts, or forward its growth? — with sundry other particulars of less or greater importance, that will occur from time to time to attentive minds.
[To THE SAME.]
EFFECT OF AIR ON THE BAROMETER — THE STUDY OF INSECTS. CRAVEN-STREET, June 11, 1760. "TIs a very sensible question you ask, How the air can affect the barometer, when its opening appears covered with wood 2 If indeed it was so closely covered as to admit of no communication of the outward air to the surface of the mercury, the change of weight in the air could not possibly affect it. But the least crevice is sufficient for the purpose; a pin-hole will do the business. And, if you could look behind the frame towhich your barometer is fixed, you would certainly find some small openIng. There are indeed some barometers in which the body of mercury at the lower end is contained in a close leather bag, and so the air cannot come into immediate contact with the mercury; yet the same effect is produced. For, the leather being flexible, when the bag is pressed by any additional weight of air it contracts, and the mercury is forced up into the tube; when the air becomes lighter, and its pressure less, the weight of the mercury prevails, and it descends again into the bag. Your observation on what you have lately read concerning insects is very just and solid. Superficial minds are apt to despise those who make that part of the creation their study, as mere triflers; but certainly the world has been much obliged to them. Under the care and management of man, the labors of the little silk-worm afford employment and subsistence to thousands of families, and become an immense article of commerce. The bee, too,