To Mary Stevenson
ALS: Library of Congress9
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, tho’ 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 Amazones, and I think the Oranoko and the Missisipi. 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 Chesapeak 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 allow’d 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 soak’d into the Earth. The Union of a Number of Springs forms a River. The Waters as they run, expos’d to the Sun, Air and Wind, are continually evaporating. Hence in Travelling one may often see where a River runs, by a long blueish Mist over it, tho’ 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 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 stopt 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 increas’d, 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 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 Chesapeak in Virginia, mentioned above; you will see, communicating with it by their Mouths, the great Rivers Sasquehanah, Potowmack, Rappahanock, York and James, besides a Number of smaller Streams each as big as the Thames. It has been propos’d 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? The Sasquehanah 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.2
But first let me mention an Experiment you may easily make your self. 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 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 Broad Cloth from a Taylor’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 warm’d 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 Colours less as they were lighter; and the quite White remain’d on the Surface of the Snow, not having entred it at all. What signifies Philosophy that does not apply to some Use?3 May we not learn from hence, that black Cloaths are not so fit to wear in a hot Sunny Climate or Season as white ones; because in such Cloaths 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 labour in the Sun, should in the East or West Indies have an Uniform of white? That Summer Hats for Men or Women, should be white, as repelling that Heat which gives the Headachs to many, and to some the fatal Stroke that the French call the Coup de Soleil? 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 Linnen within the Crown of a black Hat, as some do, will not keep out the Heat, tho’ it would if plac’d without[?] That Fruit Walls being black’d may receive so much Heat from the Sun in the Daytime, as to continue warm in some degree thro’ 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? I am, Yours affectionately,
9. Printed as Letter LVI in Exper. and Obser. (1769 edit.), pp. 469–72, 465–6; some pages numbers are repeated and the sheets are variously bound in surviving copies; (1774 edit.), pp. 479–83.
1. The ALS (which may have been the copy sent to the printer in 1769) is undated, but in Exper. and Obser. the date is given as “September 20, 1761.” This seems most improbable. At that time BF, WF, and Richard Jackson were returning from a journey through Flanders and Holland and, after a stormy crossing in a leaky sloop, just “made shift to get [to London] time enough for the Coronation” of George III, which took place on September 22. BF to DF, Utrecht, Sept. 14, 1761; WF to Sarah Franklin, London, Oct. 10, 1761, below, p. 368. The contents of the present letter show that it followed up the exchanges between BF and Polly Stevenson on tides and rivers begun in their letters of August and Sept. 13 and 16, 1760 (above, pp. 194–5, 212–17, 217–18), and probably preceded his letter of March 30, 1761, on the same subject (below, pp. 296–7). When the MS was put up for auction in 1920 the sales catalogue dated it Oct. 1, 1760, but without saying why (Stan V. Henkels Catalogue No. 1262, July 1, 1920, p. 9). This date too seems unlikely. Polly was still in Wiltshire as late as September 16, and BF and WF left London soon after the middle of the month for a trip to Cheshire, Wales, and Bristol, and with the intention of spending “some time at Bath” (above, p. 231), so there hardly seems time or opportunity for the “late Conversation” with Polly mentioned in his opening sentence, until after his return to London. Probably he was gone most if not all of October, since the first document that places him back in the city is dated November 4 (above, p. 241). November 1760 seems therefore the earliest that he could have written this letter, so it is placed here, but the correct date could have been a month or two later.
2. BF probably performed this experiment, described in detail further on in this letter, in 1732 or a little later, after he had read the discussion of the general subject in Hermann Boerhaave, Elementa chemiae, quae anniversario labore docuit, in publicis, privatisque, scholis (2 vols., Leyden, 1732), or the English translation by Timothy Dallowe, Elements of Chemistry (2 vols., London, 1735). BF evidently devised the experiment he told Polly about, made it himself, and also called it to the attention of his friend Joseph Breintnall (above, I, 114 n), for under date of Aug. 3, 1737, Breintnall wrote an account of a very similar experiment he had performed the preceding January 25, which, he said, he “was induced to make about seven years ago, and lately to repeat, from some Hints given me by Benjamin Franklin.” Logan Papers, Hist. Soc. Pa. A copy of the central part of this memorandum is in Franklin Papers, APS, dated Jan. 25, 1736–7. Both are printed in I. Bernard Cohen, “Franklin’s Experiments on Heat Absorption as a Function of Color,” Isis, XXXIV (1942–43), 404–7. Breintnall undoubtedly erred in remembering that BF had given him the “Hints” as much as seven years earlier, for the underlying principle was first published only in 1732 in Boerhaave’s work mentioned above. In his discussion the Dutch chemist had also cited, as BF does in this letter, the different speeds at which black and white cloths dry in the sun and the greater rapidity with which a “burning glass” can ignite blackened paper than white. BF’s contribution was in contriving the simple but effective experiment by which he could ascertain precisely the relative order in which the various colors absorb the solar heat. For an extended discussion, see I. Bernard Cohen, Franklin and Newton (Phila., 1956), pp. 215–22.
3. This sentence is often quoted to illustrate BF’s interest in the application of scientific knowledge to human welfare.