noticed, is 1660, where Mr. Boyle gives the following general account of them: they have no regular motion, but appear to move faster when in the middle of the sun's disc than when towards the limb. On the 27th of April he observed a spot on the eastern edge of the sun's disc, which remained on the sun for twelve days; he then speaks of its return or re-entrance on the eastern edge, which he and several others first observed sixteen days after its disappearance; they assert it to be the same spot, because the figure, colour, and bulk were the same; which I dare to affirm are very poor reasons indeed. 1st, in respect to the figure and bulk, I, in my few observations comparatively speaking, have seen many spots of the same figure; especially of so common a figure as this he now speaks of, quadrangular; and indeed have seen two on the disc at the same time, with a power of about 120 times, between which I could neither discoyer a difference in figure or bulk; and, 2dly, in respect to colour, some are lighter and some darker, but so many alike that it appears ridiculous to say a spot is known by its colour. The reason of a spot being for a greater length of time hid from our sight than visible to us, Mr. Boyle does not explain; I think it cannot be better explained than by the following figure, (see Plate XIII. fig. 4.) which I borrow from Long in his Astronomy, page 474. As the earth revolves round the sun the same way with the sun's rotation, the periodical revolution of a spot, seen from the earth, is longer than it would appear if it were viewed by an eye at rest; thus, in the figure let A, B, C, D, be the orbit of the earth, a, b, c, d, the equator of the sun; let a, be a spot seen in the middle of the disc by a spectator on the earth at A; the sun's rotation, carrying the spot round through b, c, d, according to the order of the letters, will in about 25 days bring it again to a; but, during that interval, the earth will be at B, and the middle of the disc will be at b; so that about two days more must be spent before a spectator upon the earth at C, will view the spot in the middle of the then apparent disc c. This is much the same case as that of the synodical month being longer than the periodical. The various opinions about the nature and formation of the solar spots are as follow: Some have thought that the sun is an opaque body, moun tainous and uneven as our earth, and that it is covered with a fiery luminous fluid; that this fluid is subject to ebbing and flowing after the manner of our tides; so as sometimes to leave uncovered the tops of rocks or hills, which appear like black spots: and that the penumbra around them are caused by a kind of froth. Others have imagined, that the fluid which sends us so much light and heat, contains a nucleus or solid globe, wherein, are several volcanoes, that, like Etna or Vesuvius, from time to time, cast forth quantities of bituminous matter up to the surface of the sun, and form those spots which are seen thereon; and, that, as this matter is gradually changed and consumed by the luminous fluid, the spots disappear for a time; but are seen to rise again in the same places, when these volcanoes cast up new matter. A third opinion is that the sun consists of a fiery luminous fluid, wherein are immersed several opaque bodies of irregular shapes; and that these bodies by the rapid motion of the sun, are sometimes buoyed or raised up to the surface where they form the appearance of spots; which seem to change their shapes, according as different sides of those bodies present themselves to our view. The last opinion I shall mention is, that the sun consists of a fluid in continual agitation; that, by the rapid motion of the fluid, some parts more gross than the rest are carried up to the surface of that luminary, in like manner as scum rises on the top of melted metal, or any thing that is boiling; that these scums, as they are differently agitated by the motion of the fluid, form themselves into those different shapes which we see in the solar spots; and, besides the optical change, are diminished in their apparent magnitude, recede a little from, or approach nearer to, one another, and are at last entirely dissipated by the continual rapid motion of the fluid, or are consumed thereby or absorbed therein. The optical change of the solar spots, here mentioned, is owing to their being seen upon a globe differently turned towards us: if we imagine the globe of the sun to have a number of circles drawn upon its surface, all passing through his poles, and cutting his equator at equal distances, these circles, which we may call meridians, if they were visible, would appear to us at unequal distances. Now suppose a spot were round, and so large as to reach from one meridian to another, it would appear round only when it was in the middle of that half of the globe which is towards our earth, for then we view the full extent of it in length and breadth: in every other place, it turns away from us, and appears narrower, though of the same length, the farther from the middle; and at its coming on at the eastern limb, and going off at the western limb, it appears as small as a thread, the thin edge of it being all that we then see. This apparent change of the shape in the solar spots is a plain proof that the sun is a globe. 1 am, Sir, yours, Lower Edmonton. " CHARLES H. ADAMS. |