partly from Bouguer's experiments. Hence the sun would appear like Jupiter, if removed to 131000 times his present distance.-Mem. Berl. Acad. 1739.

Milky Way.

Lambert regards the Milky way as the elliptic of the fixed stars: he thinks the greater stars belong to the solar nebula, and that the other nebulæ are confused together in the milky way.-Photometria, § 1139.

Herschel conjectures that the milky way is the projection of our nebula, and that the sun has a motion towards its node near Cepheus and Cassiopeia, 1784. In a circle of 15' diameter, 588 stars were counted: if these were at equal distances in a cone, the length of the cone must have been 497 times their distance, From calculations of this kind a figure of the nebula is drawn, showing a section passing through its poles at right angles to the line of the nodes. The right ascension of the pole is 186°, its polar distance 58°;1785. -Phil. Trans. 1784-1802. Young's Nat. Phil. II.

In 1795 Dr. Herschel found 600 stars in a circle of 15′ in diameter. Phil, Trans. 1795. He traced out a variety of double stars, which he at length followed up to the number of fifty. Cassini verified this discovery of Herschel's, but differs a little as to the colour of these stars, and enquires whether they may not be satellites. A. P. 1784.-Michell, in like manner, conjectures that some stars may move round others.Phil. Trans. 1784.

Sun.

The spots on the sun, many of which are as large, and some five or six times as large as the diameter as the earth, were first discovered by Fabricius of Wirtemberg in 1611; but their nature has been a source of perpetual controversy. Derham regarded them as clouds of volcanoes, afterwards becoming sacula. Crabtrie, as early as 1640, described them as exhalations like clouds. 'Lalande, and various others, believed them to be mountains; Wilson, in opposi. tion to Lalande, asserted them to be excavations. Herschel, Phil. Trans. 1795, represents the sun as an opaque body, probably inhabited, covered with an atmosphere in which clouds of a luminous matter are floating, and the spots as interruptions of these clouds. He believes them to exist in two strata, of which the upper only is

luminous, and the under stratum au interposition to protect the body of the sun from the luminous heat.

In Phil. Trans. 1801, Dr. Herschel endeavours to show that the variation in the heat of different years is owing to the more or less copious supply of fuel in the sun, and that it is this fuel that constitutes his spots

The motion of the sun, accompanied by the whole solar system, has also been maintained and doubted of. Mayer suggested that it' takes place towards the corona borealis. Prevost adopted the same idea. Lichtenberg was sceptical-.Herschel, id. p. 332.

Planets.

There is a material difference in the calculation of different philosophers as to the surface and temperature both of the primary and secondary planets that belong to the solar system. Thus in Chapter XVI. Mr. Baily, in his synopsis of La Place's Exposition, has calculated the proportion of light and heat existing in Mercury at rather more than six times and a half the mean light and heat of the earth; the light and heat of Venus at nearly double that of the earth; those of Mars something less than half those of the earth; those of Jupiter more than a third. Dr. Young, on the contrary, has estimated these powers prodigiously higher for Mercury and Venus, and considerably lower for Mars, Jupiter, and Saturn. His words are as follows.

"Of Mercury we know little except the length of his year, which is shorter than three of our months. Supposing all our heat to come from the sun, it is probable that the mean heat on Mercury is above that of boiling quicksilver; and it is scarcely possible that there should be any point about his poles where water would not boil. The sun's diameter would appear, if viewed from Mercury, more than twice as great as to us on the earth.

"Venus must have a climate far more temperate than Mercury, yet much too torrid for the existence of animals or vegetables, except in some circumpolar parts; her magnitude and diurnal rotation differ but little from those of the earth, and her year is only one third shorter; so that her seasons, and her day and night, must greatly resemble ours. The earth, when in opposition to the sun, must be about four times as bright as Venus ever appears to us, and must, therefore, always cast a shadow; it must be frequently, and

perhaps generally, visible in the day; and together with the moon must exhibit a very interesting object. The atmosphere of Venus is supposed to be nearly like our own, or somewhat more rare.

"The climate of Mars is as much colder than ours, as that of Venus is warmer; in other respects there is no very striking difference: the inclination of his axis to his ecliptic being nearly the same as that of the earth's axis, the changes of seasons must be nearly like our own. Dr. Herschel has observed a constant appearance of two bright spots or circles near the poles of Mars, which he attributes to the ice and snow perpetually surrounding them. It is not, however, probable that water could remain fluid in any part of Mars, and even quicksilver and alcohol would, perhaps, be frozen in his temperate climates. It is pretty certain that Mars has an atmosphere, and his dark spots seem to be occasioned by clouds: this atmosphere may, perhaps, also be the cause of the ruddy hue of his light.

"It appears to be doubtful, whether either of the three little planets newly discovered can be sufficiently solid, to give a firm footing to any material beings: we should probably weigh only a few pounds each if transported there. According to Dr. Herchel's opinion, neither Ceres nor Pallas is much larger than a good Scotch estate, although they must, sometimes, appear to each other as planets of a most respectable size. The light reflected from Ceres is of a more ruddy bue than that of Pallas; both of these planets are attended by more or less of a nebulosity, proceeding, perhaps, from copious atmospheres; and in this respect, as well as in the great inclination of their orbits, they appear to have some affinity to comets. It is tolerably certain that neither of them is 200 miles in diameter; and Juno is also probably about the same size.

"It is obvious that the most striking features of the heavens, when contemplated from Jupiter, would be the diversified positions and combinations of his satellites: their light must be faint, but yet of service; and to a traveller on the surface of this vast globe they must afford useful information, as well with respect to time as to place. Our little earth must probably be always invisible to a spectator situated on Jupiter, on account of its apparent proximity to the sun, in the same manner as a planet at half the distance of Mercury would be invisible to us. The year of Jupiter must contain nearly ten thousand of his days, and that of Saturn almost thirty thousand VOL. I.

Saturnian days. 'Besides the vicissitudes of the seven satellites revolving round Saturn, his ring must afford, in different parts of his surface, very diversified appearances of magnificent luminous arches, stretched across the heavens, especially in that hemisphere which is on the same side of the ring with the sun.

"From the Georgian planet the sun must be seen but as a little star, not one hundred and fiftieth part as bright as he appears to us. The axis of this planet being probably near to the plane of its ecliptic, it must be directed twice in the year towards the sun, and the limit of illumination must approach to the equator, so that almost every place on his surface must sometimes remain, for a great number of diurnal revolutions, in light and in darkness; the most moderate climates having one night, in their long year, equal in duration at least to several of our years: and it must be confessed that this planet would afford but a comfortless habitation to those accustomed to our summer sunshine, even if it were possible to colonize it."-Young's Nat. Phil. Vol. I.

In this difference of estimation we incline to that of Mr. Bailey. So in Chapter XVIII. we have already observed that Dr. Herschel has disproved those enormous mountains on the surface of Venus, which Schröter persuaded himself he had detected. Some astronomers have occasionally supposed that they have discovered a satellite attendant upon Venus; for an account of which the reader may turn to Mr. Short's paper, Phil. Trans. 1741, and Bode's Jahrbuch, 1777 and 1778.

In like manner Schröter calculated the moon to possess mountains four thousand toises, or nearly five miles high; and to have a twilight of such a nature as to indicate an atmosphere of three hundred toises high. Phil. Trans. 1792. Schröter. Phil. Mag. XV. Dr. Herschel, on the contrary, as we have already noticed in Chapter XIX. makes the loftiest mountains in the moon only a mile and three quarters high, and found few or no signs of a lunar atmosphère in an eclipse. The question of an atmosphere, however, is still in an unsettled state, though it is now uniformly admitted, 'that if the moon possess one at all, it is of an extreniely attenuated nature.

Riccioli however calculated the height of the lunar mountais to be far superior to those of Schröter,' and was opposed in his day by Hevelius, as Schröter has been by Herschel. These astronomers affected to divide the lunar mountains into regular orders, and to

distinguish them by separate names, in the same manner as geogra phers distinguish the mountains of our own globe. Derhamn has given a good account of their different conjectures upon these subjects in the following passage.

"By Riccioli's measures the height of what he calls Mount Si nai, or St. Katharine's Hill, is 9 Bononian miles, and that of Xaverius twelve; but according to his corrections, the former is but 8 miles, the latter 114. Which at the rate of 6020 English feet in a Bononian mile, is about 13 and 9 English miles; an height so great, considering how much the moon is less than the earth, that I cannot but think that diligent person was mistaken in his measures, and that the computations of Hevelius are much the best: who, as he was as able as any man, and made more accurate and diligent observations of the moon's face than most men ever did, so was more likely to come nearest the truth. And by his reckoning, the highest bills in the moon are but about of a German mile, and some of them but ths, and some not above an Italian mile. And considering the bulk of the moon to that of the earth, these are great eminences for the moon.

"And as the lunar mountains are of prodigious heights, so many of them are of great extent. Hevelius reckons the lunar Taurus to reach to 170 German miles; Mount Sepher 150; and the lunar Apennine above 100 German miles.

"The way how to measure the height of the mountains of the moon is not difficult, nor uncertain; which is, by observing the distance between the distant golden spots, at their first appearance (which are the tops of hills) and the enlightened part of the moon. Which distance may be computed by miles, or any other equal parts, into which we can imagine the moon's diameter divided. Hevel, Selenogr. ch. 8; Galilæi Nunc, Sider. p. 14; Riccioli Almagest. L. 4. c. 8. Schol.

"On the edge of the moon which is next the sun, I could never perceive with my best glasses any the least sign of a mountain, but all to be exactly level and smooth. Only indeed there are some certain transient roughnesses and unevennesses on the limb caused by vapours, especially when the moon is near the horizon, and in windy and some other weather. At which times the motion of the air and vapours makes a pretty crispation and rolling like waves on the moon's limb, which have the appearance of moving mountains