Томас Джефферсон - Memoir, Correspondence, And Miscellanies, From The Papers Of Thomas Jefferson, Volume 2

TO MRS. COSWAY.

Paris, October 13, 1786.

My Dear Madam,

Just as I had sealed the enclosed, I received a letter of a good length, dated Antwerp, with your name at the bottom. I prepared myself for a feast. I read two or three sentences: looked again at the signature, to see if I had not mistaken it. It was visibly yours. Read a sentence or two more. Diable! Spelt your name distinctly. There was not a letter of it omitted. Began to read again. In fine, after reading a little, and examining the signature alternately, half a dozen times, I found that your name was to four lines only, instead of four pages. I thank you for the four lines, however, because they prove you think of me; little, indeed, but better little than none. To show how much I think of you, I send you the enclosed letter of three sheets of paper, being a history of the evening I parted with you. But how expect you should read a letter of three mortal sheets of paper? I will tell you. Divide it into six doses of half a sheet each, and every day, when the toilette begins, take a dose, that is to say, read half a sheet. By this means, it will have the only merit its length and dulness can aspire to, that of assisting your coiffeuse to procure you six good naps of sleep. I will even allow you twelve days to get through it, holding you rigorously to one condition only, that is, that at whatever hour you receive this, you do not break the seal of the enclosed till the next toilette. Of this injunction I require a sacred execution. I rest it on your friendship, and that in your first letter, you tell me honestly, whether you have honestly performed it. I send you the song I promised. Bring me in return the subject, Jours heureux! Were I a songster, I should sing it all to these words; ‘ Dans ces lieux qu’elle tarde à se rendre! ’ Learn it, I pray you, and sing it with feeling. My right hand presents its devoirs to you, and sees with great indignation the left supplanting it in a correspondence so much valued. You will know the first moment it can resume its rights. The first exercise of them shall be addressed to you, as you had the first essay of its rival. It will yet, however, be many a day. Present my esteem to Mr. Cosway, and believe me to be yours very affectionately,

Th: Jefferson.

M. LE ROY DE L’ACADEMIE DES SCIENCES.

Paris, November 13, 1786.

Sir,

I received the honor of yours of September the 18th, 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 its 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 that, at present, they penetrate sometimes as far as the first mountains, which are above an hundred miles further 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 northwesterly breeze is, for the most part, felt there, while an easterly or northeasterly 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 36 1/2 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 phenomena 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 its place, becomes warmed and lightened in its turn, ascends, and is succeeded as that which went before. If the heated surface be circular, the air flows to it from every quarter, like the rays of a circle to its centre. 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 through 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 its descent, causes it to vibrate beyond the point of its 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. Its 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 its reflected rays; 3. by contact with the heated surface. A forest receiving the sun’s rays, a part of them enters 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 before-mentioned, 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, its parts are constantly mixed together; so that instead of its heat being all accumulated in its surface, as in the case of a solid, opaque body, it is diffused through its whole mass. Its surface, therefore, is comparatively cool, for these reasons; to which may be added that of evaporation. The small degree of reflection which might otherwise take place, is generally prevented by the rippled state of its 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 phenomena under consideration. The first settlements of Virginia were made along the sea coast, bearing from the south, towards the north, a little eastwardly. These settlements formed a zone, in which, though every point was not cleared of its forest, yet a good proportion was cleared and cultivated. The 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 its warm air. The breeze from the west is light and feeble; because it traverses a country covered with mountains and forests, which retard its current. That from the east is strong; as passing over the ocean, wherein there is no obstacle to its 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.