hundred feet in length, the great length being necessary in order to avoid, as far as possible, the chromatic aberration of the single lenses of which object-glasses were then made! Considering the enormous difficulties under which they labored, the results attained by these early observers are astonishing. The delineations of the planet's surface made by Huygens and Hooke were sufficiently exact to be used by modern astronomers in ascertaining the rotation period of the planet within a fraction of a second, while Cassini's observations enabled him to calculate that period with an error of less than three minutes. In fact, Huygens saw enough to suggest to his penetrating mind the existence of an analogy between the surface of Mars and that of the Earth.
In 1666 Cassini made a drawing of Mars, which is reproduced in our cut, showing in rough outline a feature of the planet's surface which has since become well known under the names of Kaiser Sea and the Hour-glass Sea, the last being suggested by its shape. Directly underneath Cassini's drawing I have placed, for the purpose of comparison, a picture of Mars made by Herschel in 1780, and showing the same sea, but with much more detail. Allowing for the difference in the position of the disk (for the two drawings were plainly not made at precisely the same period of the planet's rotation, nor at the same inclination), and also considering the great superiority of Herschel's telescope, the resemblance is sufficiently striking to show that the two observers were looking at the same feature of the planet, and that it was a permanent marking on the disk. The south polar ice-cap is conspicuous in Cassini's drawing.
A word, by the way, in regard to the "seas" and "ice-caps" of Mars. The general color of the planet is ruddy, some observers say rose-color, and this hue is plain in naked-eye observations. But the telescope shows that the disk, instead of being uniformly red, although that tint predominates, is divided into streaks and patches of varying hue. The reddish regions are regarded as being the land-surfaces of the planet, while the dusky or greenish parts are looked upon as probably oceans or seas. At the poles there are seen white caps which, inasmuch as they increase in size when it is winter and decrease when it is summer in their respective hemispheres, are regarded as the arctic and antarctic snow regions of Mars.
From the time of Herschel the study of the surface markings of Mars was prosecuted by many observers with more or less success, and Beer and Mädler, those indefatigable portrayers of celestial scenery, made a chart of Mars; but it was not until some twenty years ago that a reasonably full and satisfactory map of the red planet was produced. Then Mr. Proctor, using the drawings of the "eagle-eyed" Dawes as the basis of his work, con-
