ut it is just at
those times that we see least of her surface, because she exhibits to us
only a slender crescent. Terrestrial astronomers are accordingly very
badly placed for the study of her physical constitution. The best
observations can be made when she is situated to right or left of the
Sun, and shows us about half her illuminated disk--during the day for
choice, because at night there is too much irradiation from her dazzling
light.

These phases were discovered by Galileo, in 1610. His observations were
among the first that confirmed the veracity of the system of Copernicus,
affording an evident example of the movement of the planets round the
sun. They are often visible to the unaided eye with good sight, either
at dusk, or through light clouds.

[Illustration: FIG. 38.--Venus at greatest brilliancy.]

Venus, surrounded by a highly dense and rarefied atmosphere, which
increases the difficulties of observing her surface, might be called the
twin sister of the Earth, so similar are the dimensions of the two
worlds. But, strange as it may seem to the many admirers, who are ready
to hail in her an abode of joy and happiness, it is most probable that
this planet, attractive as she is at a distance, would be a less
desirable habitation than our floating island. In fact, the atmosphere
of Venus is perpetually covered with cloud, so that the weather there
must be always foggy. No definite geographical configuration can be
discovered on her, despite the hopes of the eighteenth-century
astronomers. We are not even sure that she rotates upon herself, so
contradictory are the observations, and so hard is it to distinguish
anything clearly upon her surface. A single night of observation
suffices to show the rotation of Mars or of Jupiter; but the beautiful
Evening Star remains obstinately veiled from our curiosity.

Several astronomers, and not the least considerable, think that the
tides produced by the Sun upon her seas, or globe in its state of
pristine fluidity, must have been strong enough to seize and fix her, as
the Earth did for the Moon, thus obliging her to present always the same
face to the Sun. Certain telescopic observations would even seem to
confirm this theoretical deduction from the calculations of celestial
mechanics.

The author ventures to disagree with this opinion, its apparent
probability notwithstanding, because he has invariably received a
contrary impression from all his telescopic observa