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To M. Dubourg.

London, July 28, 1760. * * I GREATLY approve the epithet which you give, in your letter of the Sth of June, to the new method of treating the small-pox, which you call the toric or bracing method; I will take occasion, from it, to mention a practice to which I have accustomed myself. You know the cold bath has long been in vogue here as a tonic ; but the shock of the cold water has always appeared to me, generally speaking, as too violent; and I have found it much more agreeable to my constitution to bathe in another element- mean cold air. With this view I rise al. most every morning, and sit in my chamber without any clothes whatever, half an hour or an hour, according to the season, either reading or writing. This practice is not in the least painful, but, on the contrary, agreeable ; and if I return to bed afterwards, before I dress myself, as sometimes happens, I make a supplement to my night's rest of one or two hours of the most pleasing sleep that can be imagined. I find no ill cor.sequences whatever resulting from it, and that at least it does not injure my health, if it does not in fact contribute much to its preservation. I shall therefore call it for the future a bracing or tonic bath. * * *



* * I shall not attempt to explain why damp clothes occasion colds rather than wet ones, because I doubt the fact; I imagine that neither the one nor the other contribute to this effect, and that the causes of colds are totally independent of wet, and even of cold. I propose writing a short paper on this subject, the first moment of leisure I have at my disposal. In the mean time, I can only say, that having some suspicions that the common notion, which attributes to cold the property of stopping the pores and obstructing perspiration, was ill founded, I engaged a young physician, who is making some experiments with Sanctorius's balance, to estimate the different proportions of his perspiration, when remaining one hour quite naked, and another warmly clothed. He pursued the experiment in this alternate manner for eight hours successively, and found his perspiration almost double during those hours in which he was naked. * * *


To Miss Stephenson.


Sept, 20, 176. 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. 'Tis a kind of auda

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eity 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 oar 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 Oromoko 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 op 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 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 blueish 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 ruus 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. be conceived as a moveable one, which is not only pushed some miles higher up the river by every floodtide 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.

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 stopped by the opposing food-tide, by being turned back twice in twenty-four hours, and by finding broader beds in the low dlat 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 trom 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

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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 in. clines 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 Susquehannah, Potowmack, Rappahanock, 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 ? The Susquehannah alone would seem to be sufficient for this, if it were

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