Thomas Jefferson Papers

From Thomas Jefferson to Jean Baptiste Le Roy, 13 November 1786

To Jean Baptiste Le Roy

Paris Nov. 13. 1786.


I received the honour of yours of Sep. 18.1 a day or two after the accident of a dislocated wrist had disabled me from writing. I have waited thus long in constant hope of recovering it’s use. But finding that this hope walks before me like my shadow, I can no longer oppose the desire and duty of answering your polite and learned letter. I therefore employ my left hand in the office of scribe, which it performs indeed slowly, awkwardly and badly.

The information given by me to the Marquis de Chastellux, and alluded to in his book and in your letter, was that the sea breezes which prevail in the lower parts of Virginia during the summer months, and in the warm parts of the day, had made a sensible progress into the interior country: that formerly, within the memory of persons living, they extended but little above Williamsburg; that afterwards they became sensible as high as Richmond, and at present they penetrate sometimes as far as the first mountains, which are above an hundred miles farther from the sea coast than Williamsburg is. It is very rare indeed that they reach those mountains and not till the afternoon is considerably advanced. A light North-Westerly breeze is for the most part felt there, while an Easterly, or North Easterly wind is blowing strongly in the lower country. How far Northward and Southward of Virginia this Easterly breeze takes place, I am not informed. I must therefore be understood as speaking of that state only, which extends on the sea coast from 36½ to 38° of latitude.

This is the fact. We know too little of the operations of Nature in the physical world to assign causes with any degree of confidence. Willing always however to guess at what we do not know, I have sometimes indulged myself with conjectures on the causes of the phænomena above stated. I will hazard them on paper for your amusement, premising for their foundation some principles believed to be true.

Air, resting on a heated and reflecting surface, becomes warmer, rarer and lighter: it ascends therefore, and the circumjacent air, which is colder and heavier, flows into it’s place, becomes warmed and lightened in it’s turn, ascends and is succeeded as that which went before.2 If the heated surface be circular, the air flows to it from every quarter, like the rays of a circle to it’s center. If it be a zone of determinate breadth and indefinite length, the air will flow from each side perpendicularly on it. If the currents of air flowing from opposite sides be of equal force, they will meet in equilibrio at a line drawn longitudinally thro the middle of the zone. If one current be stronger than the other, the stronger one will force back the line of equilibrium towards the further edge of the zone, or even beyond it: the motion it has acquired causing it to overshoot the zone, as the motion acquired by a pendulum in it’s descent causes it to vibrate beyond the point of it’s lowest descent.

Earth, exposed naked to the sun’s rays, absorbs a good portion of them; but being an opaque body, those rays penetrate to a small depth only. It’s surface, by this accumulation of absorbed rays, becomes considerably heated. The residue of the rays are reflected into the air resting on that surface. This air then is warmed 1. by the direct rays of the sun. 2. by it’s reflected rays. 3. by contact with the heated surface. A Forest receiving the sun’s rays, a part of them enter the intervals between the trees, and their reflection upwards is intercepted by the leaves and boughs. The rest fall on the trees, the leaves of which being generally inclined towards the horizon, reflect the rays downwards. The atmosphere here then receives little or no heat by reflection. Again, these leaves having a power of keeping themselves cool by their own transpiration, they impart no heat to the air by contact. Reflection and contact then, two of the three modes beforementioned of communicating heat, are wanting here, and of course the air over a country covered by forest must be colder than that over cultivated grounds. The sea being pellucid, the sun’s rays penetrate it to a considerable depth. Being also fluid, and in perpetual agitation, it’s parts are constantly mixed together; so that instead of it’s heat being all accumulated in it’s surface, as in the case of a solid opaque body, it is diffused thro’ its whole mass. It’s surface therefore is comparatively cool, for these reasons, to which may be added that of evaporation.3 The small degree of reflection, which might otherways take place is generally prevented by the rippled state of it’s surface. The air resting on the sea then, like that resting on a forest, receives little or no heat by reflection or contact; and is therefore colder than that which lies over a cultivated country.

To apply these observations to the phænomena under construction.

The first settlements of Virginia were made along the sea coast, bearing from South towards the North, a little Eastwardly. These settlements formed a zone in which, tho every point was not cleared of it’s forest, yet a good proportion was cleared and cultivated. This cultivated earth, as the sun advances above the horizon in the morning, acquires from it an intense heat, which is retained and increased through the warm parts of the day. The air resting on it becomes warm in proportion and rises. On one side is a country still covered with forest: on the other is the ocean. The colder air from both of these then rushes towards the heated zone to supply the place left vacant there by the ascent of it’s warm air. The breeze from the West is light and feeble; because it traverses a country covered with mountains and forests, which retard it’s current. That from the East is strong; as passing over the ocean wherein there is no obstacle to it’s motion. It is probable therefore that this Easterly breeze forces itself far into, or perhaps beyond the zone which produces it. This zone is, by the increase of population, continually widening into the interior country. The line of equilibrium between the Easterly and Westerly breezes is therefore progressive.

Did no foreign causes intervene, the sea breezes would be a little Southwardly of the East, that direction being perpendicular to our coast. But within the tropics there are winds which blow continually and strongly from the East. This current affects the courses of the air even without the tropics. The same cause too which produces a strong motion of the air from East to West between the tropics, to wit, the Sun, exercises it’s influence without these limits, but more feebly in proportion as the surface of the globe is there more obliquely presented to it’s rays. This effect, tho’ not great, is not to be neglected when the sun is in, or near, our summer solstice, which is the season of these Easterly breezes.

The Northern air too, flowing towards the equatorial parts to supply the vacuum made there by the ascent of their heated air, has only the small rotatory motion of the polar latitudes from which it comes. Nor does it suddenly acquire the swifter rotation of the parts into which it enters. This gives it the effect of a motion opposed to that of the earth, that is to say of an Easterly one. And all these causes together are known to produce currents of air in the Atlantic, varying from East to North East as far as the 40th. degree of Latitude. It is this current which presses our sea breeze out of it’s natural South Easterly direction to an Easterly and sometimes almost a North Easterly one.4

We are led naturally to ask where the progress of our sea breezes will ultimately be stopped? No confidence can be placed in any answer to this question. If they should ever pass the Mountainous country which separates the waters of the Ocean from those of the Missisipi, there may be circumstances which might aid their further progress as far as the Missisipi. That Mountainous country commences about 200 miles from the sea coast, and consists of successive ranges, passing from North East to South West, and rising the one above the other to the Alleghaney ridge, which is the highest of all. From that, lower and lower ridges succeed one another again till, having covered in the whole a breadth of 200 miles from South East to North West, they subside into plain, fertile country, extending 400 miles to the Missisipi, and probably much further on the other side towards the heads of it’s Western waters. When this country shall become cultivated, it will, for the reasons before explained, draw to it winds from the East and West. In this case, should the sea breezes pass the intermediate mountains, they will rather be aided than opposed in their further progress to the Missisipi. There are circumstances however which render it possible that they may not be able to pass those intermediate mountains. 1. These mountains constitute the highest lands within the United States. The air on them must consequently be very cold and heavy, and have a tendency to flow both to the East and West. 2. Ranging across the current of the sea breezes, they are in themselves so many successive barriers opposed to their progress. 3. The country they occupy is covered with trees, which assist to weaken and spend the force of the breezes. 4. It will remain so covered; a very small proportion of it being capable of culture. 5. The temperature of it’s air then will never be softened by culture.

At present I suppose the currents of air between the Atlantic and the Western heads of the Missisipi may be represented as in the following diagram of a horizontal section of that country.

But that when the plane country on both sides of the Missisipi shall be cleared of it’s trees and cultivated, the currents of air will be in the following directions.

Whether, in the plane country between the Mississipi and Alleganey mountains Easterly or Westerly winds prevail at present, I am not informed. I conjecture however that they must be Westerly, as represented in the first diagram: and I think, with you Sir, that if those mountains were to subside into plane country as their opposition to the Westerly winds would then be removed, they would repress more powerfully those from the East, and of course would remove the line of equilibrium nearer to the sea-coast for the present.

Having had occasion to mention the course of the Tropical winds from East to West, I will add some observations connected with them. They are known to occasion a strong current in the ocean in the same direction. This current breaks on that wedge of land of which Saint Roque is the point; the Southern column of it probably turning off and washing the coast of Brazil. I say probably because I have never heard the fact and conjecture it from reason only. The Northern column, having it’s Western motion diverted towards the North and reinforced by the currents of the great rivers Orinoko, Amazons and Tocantin, has probably been the agent which formed the gulph of Mexico, cutting the American continent nearly in two in that part. It re-crosses into the ocean at the Northern end of the gulph, and passes, by the name of the Gulph stream, all along the coast of the United States to it’s Northern extremity. There it turns off Eastwardly, having formed, by it’s eddy at this turn, the banks of New found land. Thro’ the whole of it’s course, from the gulph to the banks, it retains a very sensible warmth. The Spaniards are at this time desirous of trading to their Philippine islands by the way of the Cape of good hope: but opposed in it by the Dutch, under authority of the treaty of Munster, they are examining the practicability of a common passage thro’ the Streights of Magellan, or round Cape Horn. Were they to make an opening thro the isthmus of Panama, a work much less difficult than some even of the inferior canals of France, however small this opening should be in the beginning, the tropical current, entering it with all it’s force, would soon widen it sufficiently for it’s own passage, and thus complete in a short time that work which otherwise will still employ it for ages. Less country too would be destroyed by it in this way. These consequences would follow. 1. Vessels from Europe, or the Western coast of Africa, by entering the tropics, would have a steady wind and tide to carry them thro’ the Atlantic, thro America and the Pacific ocean to every part of the Asiatic coast, and of the Eastern coast of Africa: thus performing with speed and safety the tour of the whole globe, to within about 24°. of longitude, or 1/15 part of it’s circumference, the African continent, under the line, occupying about that space. 2. The gulph of Mexico, now the most dangerous navigation in the world, on account of it’s currents and moveable sands, would become stagnant and safe. 3. The gulph stream on the coast of the United States would cease, and with that those derangements of course and reckoning which now impede and endanger the intercourse with those states. 4. The fogs on the banks of Newfoundland,* supposed to be the vapours of the gulph stream rendered turbid by cold air, would disappear. 5. Those banks, ceasing to receive supplies of sand, weeds and warm water by the gulph stream, it might become problematical what effect changes of pasture and temperature would have on the fisheries. However it is time to relieve you from this lengthy lecture. I wish it’s subject may have been sufficiently interesting to make amends for it’s details. These are submitted with entire deference to your better judgment. I will only add to them by assuring you of the sentiments of perfect esteem and respect with which I have the honor to be Sir your most obedient and most humble servant,

Th: Jefferson

* This ingenious and probable conjecture I find in a letter from Dr. Franklin to yourself published in the late volume of the American Philosophical Transactions.

PrC (DLC); written partly with TJ’s left hand, and partly with his right (see note 4 below); at foot of text: “M Le Roy de l’Academie des Sciences.”

1An error: Le Roy’s letter was dated 28 Sep. 1786; it was on the 18th that TJ suffered the accident to his wrist and he may, therefore, have subconsciously transposed the dates in this context.

2Here and elsewhere in his letters TJ employed a space to indicate a break approximately equal in value to the break signified by a new paragraph. This custom was not unique with him, but since his handwriting was regular and disciplined, it is clear that the space was intended and it will therefore be observed in the printing of his letters, as well as in those written to him when (as in the case of William Short) it appears to be an intended usage.

3The following is deleted at this point: “In proportion as it falls short of perfect […].”

4The letter up to this point was written with TJ’s left hand; the remainder with his right.

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