« AnteriorContinuar »
vessel is sometimes placed for the experiment, lected and condensed by the coldness of the freezes into hard ice on the bason. Now I ball, from the moisture in the air, or from our know not how to account for this, otherwise breath; or whether the feather, when dipped than by supposing, that the composition is a into the ether, might not sometimes go through better conductor of fire than the ingredients it, and bring up some of the water that was separately, and, like the lock compared with under it, I am not certain; perhaps all might the wood, has a stronger power of attracting contribute. The ice continued increasing till fire, and does accordingly attract it suddenly we ended the experiment, when it appeared from the fingers, or a thermometer put into near a quarter of an inch thick all over the it, from the bason that contains it, and froin ball, with a number of small spicula, pointing the water in contact with the outside of the outwards. From this experiment one may bason; so that the fingers have the sensation see the possibility of freezing a man to death of extreme cold, by being deprived of much of on a warm summer's day, if he were to stand their natural fire; the thermometer sinks, in a passage through which the wind blew by having part of its 'fire drawn out of the briskly, and to be wet frequently with ether, mercury; the bason grows colder to the touch, a spirit that is more inflammable than brandy as, by having its fire drawn into the mixture, or common spirits of wine. it is become more capable of drawing and It is but within these few years, that the receiving it from the hand; and through the European philosophers seem to have known bason, the water loses its fire that kept its fluid; this power in nature, of cooling bodies by so it becomes ice. One would expect, that evaporation. But in the east they have long from all this attracted acquisition of fire to been acquainted with it. A friend tells me, the composition, it should become warmer; there is a passage in Bernier's Travels through and, in fact, the snow and salt dissolve at the Hindostan, written near one hundred years same time into water, without freezing. ago, that mentions it as a practice (in travel
B. FRANKLIN. ling over dry deserts in that hot climate) to
carry water in flasks wrapt in wet woollen
cloths, and hung on the shady side of the caTo Dr. Lining, at Charleston. mel, or carriage, but in the free air; whereby,
as the cloths gradually grow drier, the water On the production of Cold by Evaporation.
contained in the flasks is made cool. They LONDON, June 17, 1758. have likewise a kind of earthen pots, unglazIn a former letter I mentioned the experi- ed, which let the water gradually and slowly ment for cooling bodies by evaporation, and doze through their pores, so as to keep the that I had, by repeatedly wetting the thermo- outside a little wet, notwithstanding the con. meter with common spirits, brought the mer- tinual evaporation, which gives great coldcury down five or six degrees. Being lately ness to the vessel, and the water contained at Cambridge, and mentioning this in con- in it. Even our common sailors seem to have versation with Dr. Hadley, professor of che had some notion of this property; for I remistry there, he proposed repeating the ex- member, that being at sea, when I was a periments with ether, instead of common youth, I observed one of the sailors, during a spirits, as the ether is much quicker in evapo- calm in the night, often wetting his finger in ration. We accordingly went to his chamber, his mouth, and then holding it up in the air, where he had both ether and a thermometer. to discover, as he said, if the air had any moBy dipping first the ball of the thermometer tion, and from which side it came; and this into the ether, it appeared that the ether was he expected to do, by finding one side of his precisely of the same temperament with the finger grow suddenly cold, and from that side thermometer, which stood then at 65; for it he should look for the next wind; which I then made no alteration in the height of the little laughed at as a fancy. column of mercury. But when the thermo May not several phenomena, hitherto unmeter was taken out of the ether, and the considered, or unaccounted for, be explained ether, with which the ball was wet, began to by this property? During the hot Sunday at evaporate, the mercury sunk several degrees. Philadelphia, in June, 1750, when the ther: The wetting was then repeated by a feather mometer was up at 100 in the shade, I sat that had been dipped into the ether, when the in my chamber without exercise, only reading mercury sunk still lower. We continued this or writing, with no other clothes on than a operation, one of us wetting the ball, and an- shirt, and a pair of long linen drawers, the other of the company blowing on it with the windows all open, and a brisk wind blowing bellows, to quicken the evaporation, the mer- through the house, the sweat ran off the backs cury sinking all the time, till it came down of my hands, and my shirt was often so wet, to 7, which is 25 degrees below the freezing as to induce me to call for dry ones to put on; point, when we left off. Soon after it passed in this situation, one might have expected, the freezing point, a thin coat of ice began to that the natural heat of the body 96, added to cover the ball. Whether this was water col- the heat of the air 100, should jointly have VOL. II. ... 2 Y
created or produced a much greater degree fore checked and retarded, till we drive away of heat in the body; but the fact was, that my that atmosphere, and bring drier air in its body never grew so hot as the air that sur place, that will receive the vapour, and thererounded it, or the inanimate bodies immersed by facilitate and increase the evaporation ? in the same air. For I remember well, that Certain it is, that mere blowing of air on a the desk, when I laid my arm upon it; a chair, dry body does not cool it, as any one may sawhen I sat down in it; and a dry shirt out of tisfy himself, by blowing with a bellows on the drawer, when I put it on, all felt exceed the dry ball of a thermometer; the mercury ing warm to me, as if they had been warmed will not fall; if it moves at all, it rather rises, before a fire. And I suppose a dead body as being warmed by the friction of the air on would have acquired the temperature of the its surface? To these queries of imagination, air, though a living one, by continual sweat- I will only add one practical observation; that ing, and by the evaporation of that sweat, was wherever it is thought proper to give ease, in kept cold.”_May not this be a reason why our cases of painful inflammation in the flesh (as reapers in Pennsylvania, working in the open from burnings, or the like) by cooling the field, in the clear hot sun-shine common in our part ; linen cloths, wet with spirit, and applied harvest-time, find themselves well able to go to the part inflamed, will produce the coolness through that labour, without being much in- required, better than if wet with water, and commoded by the heat, while they continue will continue it longer. For water, though to sweat, and while they supply matter for cold when first applied, will soon acquire keeping up that sweat, by drinking frequently warmth from the flesh, as it does not evapo of a thin evaporable liquor, water mixed with rate fast enough; but the cloths wet with rum; but if the sweat stops, they drop, and spirit, will continue cold as long as any spirit sometimes die suddenly, if a sweating is not is left to keep up the evaporation, the parts again brought on by drinking that liquor, or, warmed escaping as soon as they are warmas some rather choose in that case, a kind of ed, and carrying off the heat with them. hot punch, made with water, mixed with
B. FRANKLIN. honey, and a considerable proportion of vinegar? May there not be in negroes a quicker
J. Bowdoin, in Boston, to Dr. Franklin. evaporation of the perspirable matter from their skins and lungs, which, by cooling them Concerning the Light in Sea-Water.-Read at more, enables them to bear the sun's heat the Royal Society, December 6, 1756. better than whites do? (if that is a fact, as it
November 12, 1753. is said to be ; for the alleged necessity of hav WHEN I was at the eastward, I had an ing negroes rather than whites, to work in opportunity of observing the luminous appearthe West-India fields, is founded upon it) ance of the sea when disturbed : at the head though the colour of their skins would other- and stern of the vessel, when under way, it wise make them more sensible of the sun's appeared very bright. The best opportunity heat, since black cloth heats much sooner, and I had to observe it was in a boat, in company more, in the sun, than white cloth.
I am per- with several gentlemen going from Ports suaded, from several instances happening mouth, about three miles, to our vessel lying within my knowledge, that they do not bear at the mouth of Piscataqua river. Soon after cold weather so well as the whites; they will we set off (it being in the evening) we observed perish when exposed to a less degree of it, a luminous appearance, where the oars dashed and are more apt to have their limbs frost- the water. Sometimes it was very bright, bitten; and may not this be from the same and afterwards, as we rowed along, gradually cause? Would not the earth grow much hot- lessened, till almost imperceptible, and then ter under the summer-sun, if a constant eva- reillumined. This we took notice of several poration from its surface, greater as the sun times in the passage. When I got on board shines stronger, did not, by tending to cool it, the vessel, I ordered a pail to be dipped up, balance, in some degree, the warmer effects full of sea-water, in which, on the water's of the sun's rays? Is it not owing to the con- being moved a sparkling light appeared. I stant evaporation from the surface of every took a linen cloth, and strained some of the leaf, that trees, though shone on by the sun, water through it, and there was a like appearare always, even the leaves themselves, cool ance on the cloth, which soon went off; but to our sense ? at least much cooler than they on rubbing the cloth with my finger, it was would otherwise be ? May it not be owing renewed. I then carried the cloth to the to this, that fanning ourselves when warm, light, but could not perceive any thing upon does really cool us, though the air is itself it which should cause that appearance. warm that we drive with the fan upon our Several gentlemen were of opinion, that the faces; for the atmosphere round, and next to separated particles of putrid, animal, and other our bodies, having imbibed as much of the bodies, floating on the surface of the sea, might perspired vapour as it can well contain, re- cause that appearance ; for putrid fish, &c. ceives no more, and the evaporation is there they said, will cause it: and the sea animals
which have died, and other bodies putrified of an inch ; and yet this infinitely small point therein since the creation, might afford a suf- (if you will allow the expression) affords ficient quantity of these particles to cover a light enough to make it visible four miles; or, considerable portion of the surface of the sea; rather, affords light sufficient to effect the which particles being differently dispersed, sight at that distance. might account for the different degrees of light The smallness of the animalcula is no obin the appearance above-mentioned. But this jection then to this conjecture; for supposing account seems liable to this obvious objection, them to be ten thousand times less than the that as putrid fish, &c. make a luminous ap- minimum visible, they may, notwithstanding, pearance without being moved or disturbed, it emit light enough to affect the eyes, and so to might be expected that the supposed putrid cause the luminous appearance aforesaid. particles on the surface of the sea, should This conjecture I send you for want of somealways appear luminous, where there is not a thing better. greater light; and, consequently, that the whole surface of the sea, covered with those particles, should always, in dark nights, appear
Peter Franklin, Newport, R. Island. luminous, without being disturbed. But this
On the Saltness of Sea-Water. is not the fact.
LONDON, May 7, 1760. Among the rest, I threw out my conjec It has, indeed, as you observe, been ture, that the said appearance might be caused the opinion of some very great naturalists, by a great number of little animals, floating that the sea is salt only from the dissolution on the surface of the sea, which, on being dis- of mineral or rock-salt, which its waters hapturbed, might, by expanding their fins, or pen to meet with. But this opinion takes it otherwise moving themselves, expose such a for granted that all water was originally fresh, part of their bodies as exhibits a luminous ap- of which we can have no proof. I own I am pearance, somewhat in the manner of a glow-inclined to a different opinion, and rather worm, or fire-fly: that these animals may be think all the water on this globe was originmore numerous in some places than others; ally salt, and that the fresh water we find in and, therefore, that the appearance above- springs and rivers, is the produce of distillamentioned being fainter and stronger in dif- tion. The sun raises the vapours from the ferent places, might be owing to that: that sea, which form clouds, and fall in rain upon certain circumstances of weather, &c. might the land, and springs and rivers are formed of invite them to the surface, on which, in a that rain. As to the rock-salt found in mines, calm, they might sport themselves and glow; I conceive that, instead of communicating its or in storms, being forced up, make the same saltness to the sea, it is itself drawn from the appearance.
sea, and that of course the sea is now fresher There is no difficulty in conceiving that the than it was originally. This is only another sea may be stocked with animalcula for this effect of nature's distillery, and might be perpurpose, as we find all nature crowded with formed various ways. life. But it seems difficult to conceive that It is evident from the quantities of sea-shells, such small portions of matter, even if they and the bones and teeth of fishes found in were wholly luminous, should affect our high lands, that the sea has formerly covered sight; much more so, when it is supposed them. Then, either the sea has been higher that only a part of them is luminous. But, if than it now is, and has fallen away from those we consider some other appearances, we may high lands, or they have been lower than they find the same difficulty to conceive of them; are, and were lifted up out of the water to and yet we know they take place. For in- their present height, by some internal mighty stance, the flame of a candle, which, it is force, such as we still feel some remains of, said, may be seen four miles round. The when whole continents are moved by earthlight which fills this circle of eight miles dia- quakes. In either case it may be supposed meter, was contained, when it first left the that large hollows, or valleys among hills, candle, within a circle of half an inch diame might be left filled with sea-water, which ter. If the density of light, in these circum- evaporating, and the fluid part drying away stances, be as those circles to each other, that in a course of years, would leave the salt is, as the squares of their diameters, the can- covering the bottom; and that salt coming dle-light, when come to the eye, will be afterwards to be covered with earth, from the 1,027,709,337,600 times rarer than when it neighbouring hills, could only be found by quitted the half inch circle. Now the aper- digging through that earth. Or, as we know ture of the eye, through which the light from their effects, that there are deep fiery passes, does not exceed one tenth of an inch caverns under the earth, and even under the diameter, and the portion of the lesser circle, sea, if at any time the sea leaks into any of which corresponds to this small portion of the them, the fluid parts of the water must evagreater circle, must be proportionably, that is, porate from that heat, and pass off through 1,027,709,337,600 times less than one tenth some volcano, while the salt remains, and by
degrees, and continual accretion, becomes a Am 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; B 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 Let A, C, be the head of the canal; C, D, aim out of the mine. B. FRANKLIN.
the bottom of it; D, F, the open mouth of it next the sea. Let the strait pricked line, B,
G, represent low water mark the whole Miss Stephenson.
length of the canal, A, F, high water mark. On the Bristol Waters, and the Tide in Rivers. Now if a person standing at È, and observing
at the time of high water there, that the caLondon, Sept. 13, 1760. nal is quite full at that place up to the line E, I HAVE your agreeable letter from Bristol, should conclude that the canal is equally full which I take this first leisure hour to answer, to the same height from end to end, and therehaving for some time been much engaged in fore there was as much more water come into business.
the canal since it was down at low water Your first question, What is the reason the mark, as would be included in the oblong water at this place, though cold at the spring, space A, B, G, F, he would be greatly misbecomes warm by pumping ? It will be most taken. For the tide is a wave, and the top prudent in me to forbear attempting to an- of the wave, which makes high water, as well swer, till, by a more circumstantial account, as every other lower part, is progressive; and you assure me of the fact. Iown I should ex- it is high water successively, but not at the pect that operation to warın, not so much the same time, in all the several points between water pumped, as the person pumping.-The G, F, and A, B.-And in such a length as I rubbing of dry solids together has been long have mentioned it is low water at F, G, and observed to produce heat; but the like effect also at A, B, at or near the same time with its has never yet, that I have heard, been produc- being high water at E; so that the surface of ed by the mere agitation of fluids, or friction the water in the canal, during that situation, of fluids with solids. Water in a bottle shook is properly represented by the curve pricked for hours by a mill-hopper, it is said, discover- line, B, E, G. And on the other hand, when ed no sensible addition of heat. The produc- it is low water at E, H, it is high water both tion of animal heat by exercise is therefore to at F, G, and at A, B, at or near the same time: be accounted for in another manner, which I and the surface would then be described by may hereafter endeavour to make you acquaint- the inverted curve line, A, H, F. ed with.
In this view of the case, you will easily This prudence of not attempting to give see, that there must be very little more wareasons before one is sure of facts, I learnt ter in the canal at what we call high water, from one of your sex, who, as Selden tells us, than there is at low water, those terms not being in company with some gentlemen that relating to the whole canal at the same time, were viewing, and considering something but successively to its parts. And if you supwhich they called a Chinese shoe, and dis- pose the canal six times as long, the case puting earnestly about the manner of wearing would not vary as to the quantity of water at it, and how it could possibly be put on; put in different times of the tide; there would only her word, and said modestly, Gentlemen, are be six waves in the canal at the same time, you sure it is a shoe ? —Should not that be instead of one, and the hollows in the water settled first?
would be equal to the hills. But I shall now endeavour to explain what That this is not mere theory, but conformI said to you about the tide in rivers, and to able to fact, we know by our long rivers in that end shall make a figure, which though America." The Delaware, on which Philanot very like a river, may serve to convey my delphia stands, is in this particular similar to meaning:-Suppose a canal one hundred and the cana! I have supposed of one wave: for forty miles long, communicating at one end when it is high water at the Capes or mouth with the sea, and filled therefore with sea-wa- of the river, it is also high water at Philadelter. I choose a canal at first, rather than a phia, which stands about one hundred and forriver, to throw out of consideration the effects ty miles from the sea ; and there is at the produced by the streams of fresh water from same time a low water in the middle between the land, the inequality in breadth, and the the two high waters; where, when it comes crookedness of courses.
to be high water, it is at the same time low
water at the Capes and at Philadelphia. And an hour, no ships could ride at anchor in such the longer rivers have some a wave and half, a stream, nor boats row against it. some two, three, or four waves, according to In cominon speech, indeed, this current of their length.
In the shorter rivers of this is the water both ways from the top of the wave land, one may see the same thing in part; is called the tide ; thus we say, the tide runs for instance, it is high water at Gravesend an strong, the tide runs at the rate of one, two, hour before it is high water at London Bridge; or three miles an hour, doc. and when we are and twenty miles below Gravesend an hour at a part of the river behind the top of the before it is high water at Gravesend. There wave, and find the water lower than highfore at the time of high water at Gravesend water mark, and running towards the sea, the top of the wave is there, and the water we say, the tide runs ebb; and we are before is then not so high by some feet where the the top of the wave, and find the water higher top of the wave was an hour before, or where than low water mark, and running from the it will be an hour after, as it is just then at sea, we say, the tide runs flood ; but these Gravesend.
expressions are only locally proper; for a tide Now we are not to suppose, that because strictly speaking, is one whole wave, includthe swell or top of the wave runs at the rate ing all its parts higher and lower, and these of twenty miles an hour, that therefore the waves succeed one another about twice in current, or water itself of which the wave twenty-four hours. is composed, runs at that rate. Far from it. This motion of the water, occasioned by its To conceive this motion of a wave, make a gravity, will explain to you why the water small experiment or two. Fasten one end near the mouths of rivers may be salter at of a cord in a window near the top of a high water than at low. Some of the salt wahouse, and let the other end come down to ter, as the tide wave enters the river, runs the ground; take this end in your hand, and from its top and fore side, and mixes with the you may, by a sudden motion, occasion a fresh, and also pushes it back up the river. wave in the cord that will run quite up to Supposing that the water commonly runs the window; but though the wave is pro- during the flood at the rate of two miles in gressive from your hand to the window, the an hour, and that the flood runs five hours, parts of the rope do not proceed with the you see that it can bring at most into our wave, but remain where they were, except canal only a quantity of water equal to the only that kind of motion that produces the space included in the breadth of the canal,
So if you throw a stone into a pond ten miles of its length, and the depth between of water when the surface is still and smooth, low and high water mark; which is but a you will see a circular wave proceed from fourteenth part of what would be necessary the stone at its centre, quite to the sides of to fill all the space between low and high the pond;
but the water does not proceed water mark, for one hundred and forty miles, with the wave, it only rises and falls to form the whole length of the canal. it in the different parts of its course ; and the And indeed such a quantity of water as waves that follow the first, all make use of would fill that whole space, to run in and out the same water with their predecessors. every tide, must create so outrageous a cur
But a wave in water is not indeed in all rent, as would do infinite damage to the circumstances exactly like that in a cord; for shores, shipping, &c. and make the navigawater being a fluid, and gravitating to the tion of a river almost impracticable. earth, it naturally runs from a higher place I have made this letter longer than I intendto a lower; therefore the parts of the wave ed, and therefore reserve for another what I in water do actually run a little both ways have further to say on the subject of tides from its top towards its lower sides, which and rivers. I shall now only add, that I have the parts of the wave in the cord cannot do not been exact in the numbers, because I would Thus, when it is high and standing water at avoid perplexing you with minute calculations, Gravesend, the water twenty miles below has my design at present being chiefly to give been running ebb, or towards the sea for an you distinct and clear ideas of the first princihour, or ever since it was high water there; ples. but the water at London Bridge will run flood, After writing six folio pages of philosophy or from the sea yet another hour, till it is to a young girl, is it necessary to finish such high water, or the top of the wave arrives at a letter with a compliment ?—Is not such a that bridge, and then it will have run ebb an letter of itself a compliment ?-Does it not say, hour at Gravesend, &c. Now this motion of she has a mind thirsty after knowledge, and the water, occasioned only by its gravity, or capable of receiving it; and that the most tendency to run from a higher place to a agreeable things one can write to her are lower, is by no means so swift as the motion those that tend to the improvement of her unof the wave. It scarce exceeds perhaps two derstanding?-It does indeed say this, but miles in an hour.
then it is still no compliment; it is no more If it went, as the wave does, twenty miles than plain honest truth, which is not the cha