Memoir, Correspondence, And Miscellanies, From The Papers Of Thomas Jefferson, Volume 2. Томас ДжефферсонЧитать онлайн книгу.
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.
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 course 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 its influence without those limits, but more feebly, in proportion as the surface of the globe is there more obliquely presented to its rays. This effect, though 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 rotary 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 northeast, as far as the fortieth degree of latitude. It is this current which presses our sea breeze out of its natural southeasterly direction, to an easterly, and sometimes almost a northeasterly one.
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 Mississippi, there may be circumstances which might aid their further progress, as far as the Mississippi. That mountainous country commences about two hundred miles from the sea coast, and consists of successive ranges passing from northeast to southwest, and rising the one above the other to the Allegany 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 two hundred miles from southeast to northwest, they subside into a plain, fertile country, extending four hundred miles to the Mississippi, and probably much further on the other side, towards the heads of the 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 Mississippi. 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 its air, then, will never be softened by culture.
Whether in the plain country between the Mississippi and Allegany mountains, easterly or westerly winds prevail at present, I am not informed. I conjecture, however, that they must be westerly: and I think with you, Sir, that if those mountains were to subside into plain country, as their opposition to the westerly winds would then be removed, they