tilted toward the observer to appear straight it, or its prolongation, must pass through the centre of the disk at the moment. Such, of course, is rarely the case. At times, however, the conditions are strikingly fulfilled by the great canal called the Titan. The Titan starts from the Gulf of the Titans, in south latitude 20°, and runs north almost exactly upon the 169th meridian for an immense distance. I have followed it over 2,300 miles down the disk to about 43° north, as far as the tilt of the planet's axis would permit. As the rotation of the planet swings it round, it passes the central meridian of the disk simultaneously throughout its length, and at that moment comes out so strikingly straight it seems a substantialized meridian itself.
Although each line is the arc of a great circle, the direction taken by this great circle may be any whatsoever. The Titan, as we have seen, runs nearly due north and south. Certain canals crossing this run, on the contrary, almost due east and west. There are others again, belting the disk at well-nigh every angle between these two extremes. Nor is there any preponderance, apparently, for one direction as against any other. This indifference to direction is important as showing that the rotation of the planet has no bearing upon the inclination of the canals.
But, singular as each line looks to be by itself,