44 THE POPULAR SCIENCE MONTHLY. 



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 at- 

 tained by these early observers are astonishing. The delineations 

 of the planet's surface made by Huygens and Hooke were suffi- 

 ciently 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 sur- 

 face 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 con- 

 sidering the great superiority of Herschel's telescope, the resem- 

 blance is sufficiently striking to show that the two observers were 

 looking at the same feature of the planet, and that it was a per- 

 manent marking on the disk. The south polar ice-cap is conspicu- 

 ous 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 suc- 

 cess, and Beer and Madler, those indefatigable portrayers of celes- 

 tial 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 draw- 

 ings of the " eagle-eyed " Dawes as the basis of his work, con- 



