Many observers in the seventeenth century followed up the planet with such feeble optical power as the telescopes of that epoch provided: Fontana (who made the first sketch), Riccioli and Bianchini in Italy, Cassini in France, Huygens in Holland, and later Sir William Herschel in England. It was Cassini who first made out the whitish spots or polar caps of Mars in 1666, but not until after Huygens had noted the fact that Mars turned round on an axis in a period but little longer than the earth's. Cassini followed it up later with a more accurate value; and observations in our own day, when combined with these early ones, enable us to say that the Martian day is equal to 24 hours 37 minutes 22.67 seconds, accurate probably to the hundredth part of a second. When we know that a planet turns round on an axis, we know that it has a day. When we know the direction of the axis in space or in relation to the plane of its path round the sun, we know that it has seasons: we can tell their length and when they begin and end. It did not take many years of observation to prove that the axis round which Mars turns is tilted to the plane of its path round the sun by an angle practically the same as that at which the earth's axis is tilted. So there is the immediate inference that on Mars the order and perhaps the character of the seasons is much the same as here on the earth. At least two things, however, tend to modify them. First, the year of Mars is not 365 days like ours, but 687 days. Each of the four seasons on Mars, therefore, is proportionally longer than our seasons are. Then comes the question of atmosphere--how much of an atmosphere does Mars really possess in proportion to ours, and how would its lesser amount modify the blending of the seasons into one another? All discussion of Mars and the problems of existence of life upon that planet hinge upon the character and extent of Martian atmosphere. The planet seems never to be covered, as the earth usually is, with extensive areas of cloud which to an observer in space would completely mask its oceans and continents. Nearly all the time Mars in his equatorial and temperate zones is quite clear of clouds. A few whitish spots are occasionally seen to change their form and position in both northern and southern latitudes, and they vary with the progress of the day on Mars, as clouds naturally would. But Schiaparelli, perhaps the best of all observers, thought them to be not low-lying clouds of the nimbus type that would produce rains, but rather a veil of fog, or perhaps a temporary condensation of vapor, as dew or hoar frost. But the strongest argument for an atmosphere is based on the temporary darkening or obscuration of well known and permanent markings on the surface of Mars. These are more or less frequently observed and clouds afford the best explanation of their occurrence.