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upward a little by the surrounding mass, which is heavier; just as water and oil will stand at different heights in the two branches of a syphon tube ; or just as ice, which is lighter than water, is made to rise a little higher, on that account, when placed in water.
If the earth rested immovably upon a fixed support, there would be a tide, or rising of the waters, only on the side toward the moon. But the great body of the earth is just as free to move as a single particle of the ocean, and if suffered to yield to the moon's attraction, would be carried just as fast. Hence, for the same reason that a particle of water on the side of the earth toward the moon is drawn away from the centre, or has its downward tendency diminished, so the solid earth itself is drawn away from the mass of waters, on the side of the earth farthest from the moon. It is the difference of attraction, in both cases, between the surface and the centre, which causes the lightness of the waters, and the consequent elevation. It will be seen, therefore, that, taking the whole earth into view, there are always two high tides diametrically opposite to each other and two low tides also, midway between the high ones. The high tides are two great waves or swells, of small height, but extending each way through half a right angle. These waves follow the moon in its monthly motion round the earth, while the earth, turning on its axis, causes any given place to pass through each of these swells and the intervening depressions, in a lunar day, or 24h. 50m.
What we have said with respect to the moon's influence in disturbing the level of the ocean, may be applied also to that of the sun ; only, in the case of the sun, although its absolute action is about double that of the moon, yet, on account of its very great distance, the relative action upon the surface of the earth, compared with that at the centre, is about one third as great as that of the nioon. At new and full moon, when the sun's and moon's actions conspire, the tides are highest, and are called spring tides, But at the first and last quarters of the moon, the action on one body tends to counteract that of the other, and the tides, both at ebb and flow, are smallest, and are called neap tides.
We have supposed the highest tides to happen at new and full moon, and the lowest at the quarters. But the waters do not yield instantly to the action exerted upon them; the greatest effect takes place some time after the attractive influence has passed its point of greatest power. Thus the spring and neap tides actually occur about a day and half after the times above indicated. So, also, for a similar reason, the real time of high water, in the daily tides, happens about three hours after the moon has passed the meridian.
It wil be perceived, from what has been said, that the sun's and moon's influence will vary with a change of distance, being greatest when the ato tracting body is nearest, and vice versa,
The phenomena of the tides are modified, moreover, by the situation of the sun and moon with respect to the equator, and the particular latitude of the observer. When, for instance, the moon passes near the zenith of the observer, supposed to be in one of the temperate zones, the opposite high tide will be in the same latitude on the other side of the equator ; consequently, under the above circumstances, the high tide, when the moon is above the horizon, exceeds the high tide when the moon is below the horizon; and at a point in the direction of the nearest pole, 50° from the place where the moon is vertical, there will be only one tide in 24 hours.
The different heights to which the same tide rises in places but little distant froin each other, depend upon local circumstances; as the particular form of the coast, the meeting of currents, &c. Where a bay grows narrower and narrower, like a tunnel, as it runs up from the ocean into the land, the swell of water must rise higher, as the passage becoines more contracted in breadth. Thus in the Bay of Fundy, which answers to this description, and is of great extent, the tide sometimes rises to the height of 70 feet.
It is frequently asked, why there are not tides in the inland seas and larger lakes. If we observe, upon an artificial globe, the very small space occupied by the largest bodies of water of this description, we shall readily perceive, that there can be no appreciable difference in the action of the moon upon so small a portion of the earth's surface; the whole of the lake or sea, therefore, becoines lighter when the moon is over it, and there is po heavier mass of water 90° distant to force it above its natural level.
IX. SPOTS ON THE SUN.
The sun's spots have now been observed and speculated upon, for more than two hundred years. There is nothing like regularity in their form and appearance. They are as various in magnitude, and as diversified in figure, as the clouds of our atmosphere, and some of them are as fleeting. Sometimes twenty, thirty, and even fifty may be seen at once ; sometimes none are to be found for a number of years. They have been known to occupy an extent, equal to a fifth of the sun's diameter in length, and a twentieth in breadth.
Some spots are darker, and others brighter, than the rest of the disc. They are often found to succeed each other alternately in the same place. The dark spot consists generally of two parts, a centre and a border, or nucleus and umbra, as they are called. The centre is of a darker shade than the border, and the border is darkest towards its outer edge, and is distinguished from the deeper colored nucleus, by a well defined and very marked difference of light. But a circumstance of the utmost importance, as indicating the nature of these spots, is, that they are all. in motion, and all move with the same rapidity, and in the same direction, and if they continue long enough, they all return in nearly the same time. They are about fourteen days in passing over the sun, from east to west apparently, and in the same time reappear again in the east. Few remain long enough to make an entire revolution. One has been observed during nearly three complete periods. It appears also that these spots, at particular seasons of the year, move over the sun in straight lines, at all other times in lines more or less curved; and the paths described by different spots, observed at the same time, are always parallel to each other, and always have their curvature and position determined by the season.
These few facts will enable us to judge of the value of most of the hypotheses that have been proposed respecting the nature of the phenom. ena in question.
If our lakes Superior, Ontario, Erie, &c. were visible to a spectator at the sun, they would exhibit an appearance very similar to that which the solar spots do to us. They would not appear to pass over the centre of the earth's disc; they would all move across in the same time. They would describe straight lines, when our days and nights are equal, about the middle of March and middle of September. At all other times, their paths would be curved, and most so, at the time of our longest and shortest days. Besides, they would appear broadest when near the middle point of their passage ; and at their entering and going off, they would be contracted in breadth, their length, in the direction parallel to the sun's limb, being undiminished.
We conclude, therefore, from the facts that have been stated, that the sun's spots adhere to his surface, and that their motion is produced by the motion of the sun upon an axis not quite perpendicular to the ecliptic. They cannot be Mercury or Venus, for these planets are only a few hours in passing over the sun, and they always appear round, and move apparently in straight lines. They cannot consist of a collection of small planets nearer the sun, for similar reasons. Their motion is too slow, and they ought not to be so long between us and the sun, as they are in describing the rest of their orbits. If half of the orbit described, is comprehended within the sun's breadth, it is a sufficient proof that it coincides with his surface. Their dilating and contracting also, according as they are near the centre or border, seem to be a natural consequence of such a coincidence. This circumstance, moreover, seems not very favorable to the supposition, that these appearances proceed from elevated objects, as mountains or rocks, rising above the luminous matter of the sun. We should expect, were this the case, that they would exhibit a greater breadth and more of a triangular form, when viewed in a direction perpendicular to their basis, and that we should have the same spot return, after a certain interval of time, presenting the same appearance, and occupying the same place. But nothing of this kind is to be found in the history of these appearances.
There was a remarkable spot in 1769, which appeared to be depressed below the surface of the sun. As it approached the limb, the umbra, or shadowy margin nearest the centre of the sun, disappeared first. On its return to the other limb, the other margin, being nearest the centre, was · invisible. It was hid apparently by the intervening portion of the sun's body. As the spot advanced upon the disc, it came into view. Sir W. Herschel observed many appearances of this kind, and was fully persuaded, that these dark specks were below the surface of the sun. He has given views and observations, which tend very much to confirm this opinion. He undertook to measure the sides of the depression, and to determine its depth. He thought that the phenomena of the spots arise, not from excavations in the body of the sun, but from openings in his atınosphere; that the solid substance of the sun is opaque, like the planets, and that, like the planets, it is surrounded by an 'atmosphere ; that this atmosphere is transparent to the height of about 2000 miles, and gives support at this elevation to a stratum of dark clouds, on which, as the outer substance of the sun, rests the flood of luminous matter, which presents itself to the surrounding planets. This light is supposed to be produced by the combustion of gases, which are generated below, and which in their ascent drive away the clouds and billows of flame that float upon them, and thus give us a glimpse both of the opaque surface of the sun, and of the cloudy stratum above it, and that these form the nucleus and umbra of a spot. Our earth probably presents similar appearances to the inhabitants of the
Where it is covered with clouds, it will exhibit a uniform brightness; where there are breaks and interruptions, the naked body of the earth will be seen of a darker shade, on account of those clouds, which intercept much of the light. The side of the opening also would present themselves alternately, as the earth revolved on its axis, by which means their depth might be ascertained, as Herschel ascertained the depth of the openings in the solar clouds.
The sun's spots, then, according to this hypothesis, are chasms in his atmosphere, occasioned by ascending currents of gaseous fuel, and they are succeeded by faculæ, as they are called, or bright spots, on account of this additional supply of combustible matter, which, it inay be supposed, is most completely on fire soon after the opening has closed. But how is the sun ordinarily furnished when there are no spots ? The gas may be more diffused, and by ascending in smaller quantities, may produce no sensible disturbance of the luminous fluid. Besides, there are probably openings, that are too small to be seen, and the sun may never be free from them. It is only when their absolute magnitude is very great, that they become an object of any attention.
Herschel observed, that the luminous matter of the sun, when viewed with his best telescopes, is far from preserving always the same aspect. It is sometimes even and tranquil, and sometimes it is thrown up into ridges, and appears to be agitated, like the sea in a storm. The changes, when in this state, are often very rapid, small openings are formed and closed in a few minutes, and clouds are seen passing with a rapidity, that considerably alters their situation in the course of an hour. These fluctuations are more particularly observed during the time of large and frequent spots. Hence Herschel inferred, that there is a variable emission of light and heat, intimately connected with the appearance and disappearance of spots, and that seasons of uncommon heat and cold, of fertility and barrenness, so far as they depend upon the supply of heat, are to be traced not so much to accidental causes near at hand, as to the inconstancy of the fountain. We are like plants in a green-house, that are healthy and vigorous, or chilled with the frost, according as the flues are well or ill attended to. We depend for the very means of subsistence, as well as for all the comforts of climate, upon operations, that are going on ninety millions of miles off; upon the more or less rapid compositions and decompositions, that are taking place in this great laboratory of nature.
As the sun's spots move across the disc all in the same direction, and in the same time, and are as long behind the sun as they are before it, we infer that they belong to the surface of the sun, and are carried about by the sun's rotation. From the position of the paths described by the spots, we are able, moreover, to calculate the position of the axis on which the sun turns. It is not exactly perpendicular to the ecliptic, but inclined about 6o. As the spots are about 14 days in passing across the disc, it will be readily seen that the time of an apparent rotation is about 28 days. We say apparent, because, as the earth is carried round the sun in the same direction, a spot must describe as much more than a real revolution, as the earth has moved in the same time. Whence, to a spectator at rest, the sun would complete a revolution in between 25 and 26 days.
X. ROTATION, &c. OF THE PLANETS.
It is a curious fact that the motions of the planets, both in their annual course round the sun, and in their rotations on their axes, are nearly in the same direction, namely, from west to east. The same remark is applicable, also, to the satellites, so far as our knowledge extends, with the exception of those of Uranus or Herschel.*
* Herschel's six satellites have their orbits nearly perpendicular to the ecliptic. As to their rotation, and that of six of Saturn's satellites, and that of the new planets, we have no information whatever.