that we possess of the planet Venus, showed the existence of several mountains on that planet, the height of some of which he estimated to amount to twenty-two miles. There were three which he estimated: the first at nineteen miles, or five times the height of Chimborazo; the second at eleven and a half miles; and the third at ten and three quarters miles.

INFLUENCE OF THE SUN AT MERCURY AND VENUS.

The distance of the earth from the sun being greater than that of Mercury in the ratio of 100 to 39, or nearly 5 to 2, the apparent diameter of the sun as seen from Mercury will be greater than as seen from the earth in the same ratio. If E represent the apparent magnitude of the sun as seen from the earth, M will represent it as seen from Mercury.

The intensity of the sun's light being in the proportion of the area of its apparent disk, will be greater at Mercury than at the earth in the ratio of 25 to 4, or nearly as 6 to 1. If the heat depended solely on the sun's rays, it would

be in the same proportion greater than at the earth, but this may be modified by many causes in operation on the planet and in its atmosphere.

The distance of the earth from the sun is greater than that of Venus in the ratio of 10 to 7 nearly, and consequently the apparent diameter of the sun as seen from Venus will be greater in the same ratio than as seen from the earth. If E represent the apparent magnitude of the sun as seen from the earth, V will represent its apparent magnitude as seen from Venus.

The intensity of the sun's light at Venus will be about twice its intensity at the earth.

The existence of an extensive twilight in these planets has been well ascertained. By observing the concave edge of the crescent which corresponds to the boundary of the illuminated and dark hemispheres of the planets, it is found that the enlightened portion does not terminate suddenly, but there is a grad

ual fading away of the light into the darkness, produced by the band of atmosphere illuminated by the sun which overhangs a part of the dark hemisphere, and produces upon it the phenomena of twilight.

When we examine the dark hemisphere of the planet Venus, there is observed upon occasions a faint reddish and grayish light, which is visible on parts too distant from the illuminated hemisphere to be produced by the light of the sun. It is supposed that these effects are indications of the play of some atmospheric phenomena in this planet similar to the aurora borealis.

OF THE PLANET MARS.

Proceeding outward in the solar system from the sun, the first planet which we find revolving beyond the earth and including the annual path of the earth within its periodical course is the planet MARS. This body makes its revolution round the sun at a distance of nearly one hundred and fifty millions of miles from that luminary, and completes its revolution in six hundred and eightysix days, or a little less than two years.

When the earth is between Mars and the sun, the distance of the planet from the earth is less than fifty millions of miles, and as it is then seen in the meridian at midnight, the circumstances are extremely favorable to telescopic observation. Although its distance from the earth at that epoch is greater than that of Venus when near inferior conjunction, yet as Venus in that position has her dark hemisphere turned to the earth, while the enlightened hemisphere of Mars is turned fully toward us, the observations made on the latter are more satisfactory.

The diameter of Mars is about half that of our globe, and it has been found by the observations of Arago that its polar diameter is little less than its equatorial, and that consequently, like the earth, it is an oblate spheroid.

As the planet includes the orbit of the earth within its periodical course round the sun, the hemisphere which it presents to the sun is always very nearly, although not exactly, presented to the earth; the consequence of which is that Mars is always seen with a full phase, or very slightly gibbous. It has the appearance of a reddish star

DIURNAL ROTATION OF MARS.

On examining with a sufficiently powerful telescope the disk of Mars, it is found to be characterized by features of lights and shadows, like those which prevail on the other planets. These were observed at a very early period in the progress of astronomical discovery. There are diagrams given in the first volume of the "Philosophical Transactions," showing telescopic views of this planet.

By attentively watching these marks, they have been observed to move in parallel lines east and west-to disappear at one side of the disk, and to reappear after equal intervals at the other side. Hence it was discovered at a very early epoch by CASSINI that Mars has a diurnal motion upon its axis in a time very little different from that of the earth. Cassini's estimation of the time of rotation of this planet was twenty-four hours and forty minutes. A more accurate estimate proves it to be twenty-four hours, thirty-nine minutes, and twenty-one seconds. The axis on which it turns, and which is perpendicular to the lines in which the marks on the disk more, is at an angle of When it is rememabout thirty degrees from the perpendicular to its orbit. bered that the earth's axis is inclined at an angle of twenty-three and a half degrees, and that it is this inclination which produces the succession of sea

sons, and which divides the earth into zones and climates, it will be easily inferred that the same phenomena prevails in Mars-the limits of the seasons being little more extreme than those which prevail in the earth.

ATMOSPHERE OF MARS.

The existence of an atmosphere upon Mars is proved by the gradual diminution which the light of a star suffers as his disk approaches it, and by the variable character of the lights and shadows apparent upon the disk. The ruddy appearance of the planet has been explained by the supposition of an atmosphere of great density around it; but more accurate telescopic observations have led Herschel and others rather to incline to the opinion that this redness must be ascribed to a peculiar color prevailing on the surface of the planet, like that of the red sandstone districts upon the earth. A slight appearance of belts has always been noticed on this planet, which affords another indication of an atmosphere, as will be more clearly understood when the belts of Jupiter and Saturn shall be explained.

PHYSICAL CONSTITUTION OF MARS.

Telescopic inquiry has been directed to determine the physical condition of this planet, and with a degree of success greater perhaps than that which has attended similar inquiries respecting any other body in the solar system, except the sun and moon. Sir William Herschel, and after him his son, Sir John Herschel, ascertained the form and position of a variety of the features of light and color on the disk; but it has been reserved for the Prussian astronomers, BEER and MADLER, to carry this inquiry to a much greater degree of detailed accuracy.

Sir John Herschel made a series of observations on Mars within the last fourteen years, and supplied a telescopic drawing of one hemisphere of the planet. We annex a figure exhibiting this sketch.

He stated that the outlines here exhibited were found to be permanent and unvariable, and must therefore be regarded as geographical and not atmospheric features. It is true that they were not always visible, being sometimes obscured, or varied by what seems to be clouds; but when visible they were always the same. Some portions appeared of a reddish color, while others had a greenish tint. He supposes the red portions to be land whose geological character im