Dr. Herschel then solves a general pro blem for computing the length of the visual ray : that of the telescope, which he uses, will reach to stars 497 times the distance of Sirius. Now, according to the Doctor's reasoning, Sirius cannot be nearer than 100,000x 194,000,000 miles ; therefore his telescope will, at least, reach to 100,000 x 194,000,000 x 497 miles. And Dr. Herschel says, that in the most crowded part of the milky way he has had fields of view that contained no less than 5813 stars, and these were continued for many minutes, so that, in a quarter of an hour, he has seen 116,000 stars pass through the field view of a telescope of only 15' aperture : and at another time, in 41 minutes, he saw 258,000 stars pass through the field of his telescope. Every improvement in his telescopes has discovered stars not seen before, so that there appears no bounds to their number, or to the extent of the universe.
The sun, like many other stars, has pro bably a progressive motion, directed towards the constellation Hercules. Dr. Herschel, on this subject, observes, that the apparent proper motions of 44 stars out of 56 are nearly in the direction which would be the result of such a real motion of the solar system ; and that the bright stars Arcturus and Sirius, which are brobably the nearest to us, have, as they ought, according to this theory, the greatest apparent motions. Again, the star Castor appears, when viewed with a telescope, to consist of two stars, of nearly equal magnitude ;and though they have both an apparent motion, they have never beeo found to change their dis tance with respect to one another a single second, a circumstance which is easily understood, if both their apparent mo tions are supposed to arise from the real motion of the sun.
The sun revolves on his axis in 23d 10s, with respect to the fixed stars ; this axis is directed towards a point, about half way between the pole star and Lyra, the plane of the rotation being inclined a ante more than 7° to that in which the earth revolves. The direction of this motion is from west to east. All the tations of the different bodies which com solar system, as far as they have been ascertained, are in the same direc tion, and likewise all their revolutions, excepting those of some of the comets, and those of some of the satellites of the Herschel planet.
The time and the direction of the sun's rotation are ascertained by the change of the situation of the spots, which are usually visible on his disc, and which some astronomers supposed to be eleva tions, and others to be excavations in the luminous matter covering the sun's sur face. These spots are frequently ob
served to appear and disappear, and they are in the mean time liable to great varia tions, though they are generally found about the same points of the sun's sur face. IL Lalande supposes them to be parts of the solid body of the sun, which by some agitations of the luminous ocean with which he conceives the sun to be surrounded, are left nearly or en tirely bare. Dr. Wilson and Dr. Herschel are disposed to consider this ocean as consisting rather of a flame than of a liquid substance ; and Dr. Herschel, in an ingenious paper, attributes the spots to the emission of an zriform fluid, not yet in combustion, which displaces the gene ral luminous atmosphere, and which is afterwards to serve for fuel for supporting the process ; hence he supposes the ap pearance of copious spots to be indica tive of the approach of warm seasons on the surface of the earth, a theory which he has attempted to maintain by historical evidence. The exterior luminous at mosphere has an appearance somewhat mottled ; some parts of it, appearing brighter than others, have been called faculz ; but Dr. Herschel distinguishes them by the names of ridges and nodules. The spots are usually surrounded by margins less dark than themselves, which are called shallows, and which are con sidered as parts of an inferior stratum, consisting of opaque clouds, capable of protecting the immediate surface of the sun from the excessive heat produced by combustion in the superior stratum, and perhaps rendering it habitable to animat ed beings.
To which Dr. Young replies, if we in quire into the intensity of the heat which must necessarily exist wherever this com bustion is performed, we shall soon be convinced that no clouds, however dense, could impede its rapid transmission to the parts below. Besides, the diameter of the sun is 111 times as great as that of the earth ; and at its surface, a heavy body would fall through no less than 450 feet in a single second ; so that if every other circumstance permitted human be ings to reside on it, their own weight would present an insuperable difficulty, since it would become thirty times as great as upon the surface of the earth, and • man of moderate size would weigh above two tons.