Other markings are variable. Since the planet rotates on its axis, the positions of its poles and equator are known, its equator being inclined to its orbit at an angle of 24° 50', while the angle in the case of the Earth is 23° 27'. The times when its seasons begin and end are therefore known; and it is found that the spring of its northern hemisphere lasts 199 of our {63} days, the summer 183, the autumn 147, and the winter 158. Round the pole in winter a broad white cap forms, which begins to shrink as spring comes on, and may entirely disappear in summer. No corresponding changes have been observed on the Moon, but it is easy to find an analogy to them on the Earth. Round both our poles a great cap of ice and snow is spread--a cap which increases in size as winter comes on, and diminishes with the advance of summer--and it seems a reasonable inference to suppose that the white polar caps of Mars are, like our own, composed of ice and snow. From time to time indications have been observed of the presence on Mars of a certain amount of cloud. Familiar dark markings have, for a short time, been interrupted, or been entirely hidden, by white bands, and have recovered their ordinary appearance later. With rotation on its axis and succession of seasons, with atmosphere and cloud, with land and water, with ice and snow, Mars would seem to be a world very similar to our own. This was the general opinion up to the year 1877, when SCHIAPARELLI announced that he had discovered a number of very narrow, straight, dark lines on the planet--lines to which he gave the name of "canali"--that is, "channels." This word was unfortunately rendered into English by the word "+canals+," and, as a canal means a waterway artificially made, this mistranslation gave the idea that Mars was inhabited by intelligent beings, who had dug out the surface of the planet into a network of canals of stupendous length and breadth.