which, together with those of a spot discovered by Schrbter, gave the same period of rotation. Schrb ter states that Mercury s form is spherical, exhibiting no sensible compression ; that his equator is very considerably inclined to his orbit, and the differences of his days and seasons must consequently be very great ; and that there are mountains on his surface which cast very long shadows, and of which the height bears a greater proportion to the diameter of the planet than those of the Earth, the Moon, or even of Venus. The height of Chimborazo is 10 7 of the radius of the earth ; one of the mountains in the moon has been estimated at -^j- of her radius; the highest in Venus at y^-j- ; and one in Mercury at y-^. But recent observations render it exceedingly doubtful how far Schrbter s observations can be trusted. There are no observations to prove decisively whether Mercury is sur
rounded by an atmosphere.
Venus, the most beautiful object in the heavens, is about 7510 English miles in diameter, and is placed at the distance of 66 millions of miles from the sun. Although the oscilla tions of this planet are considerably greater than those of Mercury, and she is seldom invisible, yet on account of the uniform brilliancy of her disk, it is extremely difficult to ascertain the period of her rotation. Dominic Cassini, after having long fruitlessly attempted to discover any object on her surface so well defined as to enable him to follow its motions, at length, in 1667, perceived a bright spot, distant from the southern horn a little more than a fourth part of the diameter of the disk, and near the eastern edge. By continuing his observations of this spot, Cassini concluded the rotation of Venus to be performed in about 23 hours ; but he does not seem to have considered this conclusion as deserving of much confidence. In the year 1726 Bianchini, an Italian astronomer, made a number of similar observations for the same purpose, from which he inferred that the rotation of the planet is performed in 24 days 8 hours. The younger Cassini has shown, however, that the observations of Bianchini, as well as those of his father, could be explained by a rotation of 23 hours and 21 or 22 min. whereas the rotation of 24 days 8 hours cannot be reconciled with the appearances observed by the elder Cassini. The determination of Cassini was regarded by astronomers as the more probable of the two, particularly as Bianchini was not able to make his observations in a connected manner, because a neighbouring building inter cepted his view of the planet, and obliged him to transport his telescope to a different situation. The question of the rotation of Venus was again attacked by Schroter, who found it to be performed in 23 hours 21 min. 19 sec. Each of the three observers found the inclination of the axis of rotation to the axis of the ecliptic to be about 75. Much doubt, however, still exists with respect to the value of this element which De Vico sets at little more than 50.
Schrbter s observations of this planet were principally directed to a mountain situated near the southern horn. The bine which joins the extremities of the horns is always a diameter ; and the horns of the crescent of a perfect sphere ought to be sharp and pointed. Schroter remarked that this was not always the case with regard to the horns of Mercury and Venus. The northern horn of the latter always preserved the pointed form, but the southern occa sionally appeared rounded or obtuse, a circumstance which indicated that the shadow of a mountain covered the part ; but beyond the horn he remarked a luminous point, which he supposed to be the summit of another mountain, illuminated by the sun after he had ceased to be visible to the rest of that hemisphere. Now, in order that the horn of the crescent may appear obtuse in consequence of the shadow of a mountain falling upon it, and another moun tain may present a luminous point, the two mountains must be at the same time both at the edge of the disk and on the line separating the dark from the enlightened part of the planet. But this position cannot be of long continuance. Rotation will cause the bright point to rise into the enlightened part, or sink into the dark hemisphere, and in either case the mountain will cease to be visible. If, how ever, the rotation ia completed in 23 hours 21 min., tho mountain will appear 39 min. sooner than it did on the previous day. Hence it is possible to obtain several consecutive observations, from which an approximate value of the period may be found ; and this being once obtained, it may be rendered still more exact by observations separated from each other by a longer interval. Thus Schroter found that an interval of 20 days 1 1 hours 15 min. between two appearances of the mountain being divided by 23 hours 21 min. gave 21 -04 revolutions ; and that intervals of 121 days 14 hours 25 min., 142 days 1 hour 40 min., 155 days 18 hours 11 min., divided each by 23 hours 21 min., gave 125-01, 146-02, 160-09 revolutions respectively. Ltt these comparisons prove that the revolution of 23 hours 21 min. is somewhat too short. They ought to have given 21, 125, 146, and 160 revolutions exactly, supposing the observa tions to have been perfectly accurate. On dividing the intervals by 21, 125, 146, and 160 respectively, the quotients will be each the time of a revolution very nearly ; and by taking a mean of the whole, the most probable result at least will be obtained. In this manner Schrbter found the period of rotation already stated, namely, 23 hours 21 min. 19 sec.
Since the time of rotation of Mercury and Venus is nearly equal to that of the earth, the compression of these planets at the poles, which results from the centrifugal force, ought also to be nearly in the same proportion. But at the distance of the earth the compression must be imperceptible even in the case of Venus ; for, supposing it to amount to -g-^-g-, the difference between the radius of her poles and that of her equator would only amount to a tenth of a second as seen from the earth.
and 1769 a sort of penumbral light was observed round the planet by several astronomers, which was occasioned, without doubt, by the refractive powers of her atmosphere. Wargentin remarked that the limb of Venus which had gone off the sun showed itself with a faint light during almost the whole time of emersion. Bergman, who observed the transit of 1761 at Upsal, says that at the ingress the part which had not come upon the sun was visible, though dark, and was surrounded by a crescent of faint light ; but this appearance was much more remarkable at the egress ; for as soon as any part of the planet had disengaged itself from the sun s disk, that part was visible with a like crescent, but brighter. As more of the planet s disk disengaged itself from that of the sun, the part of the crescent farthest from the sun grew fainter, and vanished, until at last only the horns could be seen. The total immersion and emersion were not instantaneous; but as two drops of water, when about to separate, form a ligament between them, so there was a dark shade stretched out between Venus and the sun; and when this ligament broke, the planet seemed to have got about an eighth part of her diameter from the limb of the sun. The numerous accounts of the two transits which have been published abound with analogous observations, indicating the exist ence of an atmosphere of considerable height and density. During the transit of 1874 these appearances were so satis factorily seen, that no doubt can remain as to their reality. Schroter calculated that the homontal refraction of the atmosphere of Venus must amount to 30 34", differing little from that of the terrestrial atmosphere. A twilight
which he perceived on the cusps afforded him the data from