It will be noticed next that in all the mean cartouches the gradient is greater after the minimum than before it. The curves fall gently to their lowest points and rise more steeply from them. Such profile indicates that the effects of a previous force were slowly dying out down to the minimum and that then an impulse started in to act afresh. This explains the attitude of the canals that died out. In them the effect of the old force shows as in the others, but no impulse came in their case to resuscitation.
It seems possible to trace this force to an origin at the south. For beginning with the north sub-tropic zone the gradient on the left shows less and less steep southward to the south sub-tropic zone. Such a dying-down swell is what should be looked for in an impulse which had travelled from the south northward, since the wave would affect the more northern zones last, and less of a calm period would intervene between the two impulses from opposite poles.
The cartouches, then, state that the canals began to develop after the greatest melting of the polar cap had occurred; that this development proceeded down the latitudes to the equator, and then not stopping there advanced up the latitudes of the other hemisphere. In the next place they show that in the arctic region the development was arrested and devolution or decay set in as it began to get cold there, the most northern canals being affected first. Finally, that a similar wave of evolution had occurred from the opposite pole some time before and had then passed away. And this evidence of the cartouches is direct, and independent of any theory.
CHAPTER XXIV
CANAL DEVELOPMENT
Individually Instanced
As an interesting instance of the law of development we may take the career of the Brontes during this same Martian year; the Brontes witnessing individually to the same evolutionary process that the canals collectively exhibit.
The Brontes is one of the most imposing canals upon the planet. It is not so much its length which renders it a striking object, though this length is enough to entitle it to consideration, being no less than 2440 miles. Its direction is what singles it out to notice, for it runs almost north and south. For this reason it swings into a position to hold the centre of the stage for a time with the precision of a meridian, as the planet’s rotation turns its longitude into view. The points which it connects help also to add to its distinction. For the Sinus Titanum at its southern end and the Propontis at its northern are both among the conspicuous points of the disk. The latter is but twelve degrees farther east than the former, while it is sixty-six degrees farther north. This long distance,—from nearly the line of the tropics in the southern hemisphere to mid-temperate regions of the northern,—the canal runs in an absolutely straight course.
Its north and south character commends it for any investigation of canal development, since it runs in the general direction that development takes. Its great latitudinal stretch further fits it for a recorder of changes sweeping down the disk; so that both in direction and length it stands well circumstanced for a measure of latitudinal variations. The fact that it is usually a fairly conspicuous canal does not detract from its virtue in this respect. It was first recognized at Flagstaff in 1894. But once realized, so to speak, it was possible to identify it with a canal seen by Schiaparelli and supposed by him to be the Titan; indeed, it played hide and seek with that canal throughout his drawings. In 1894 both it and the Titan were so well seen that its separate existence was unmistakable, causing it to be both recognized and named. It is, like the Titan, one of the sheaf of canals descending the disk from the Sinus Titanum, and lies just to the east of the Titan in the bunch. In 1896 it was also prominent; and at both these oppositions most so from its southern end, its northern one being more or less indefinite, especially in 1894.
In 1901 it was not the same. Instead of being the conspicuous canal it had been in earlier years, it was now so faint as with difficulty to be made out. It remained so to the close of observations. It was now under suspicion. Its behavior in 1896-1897 had led to the supposition that not only were seasonal changes taking place in it, but that those changes were such as to point to a law in the case with which its conduct in 1901 fayed in. The suspicion did not, however, become a certainty till the opposition of 1903. The length of time during which the disk was then kept under scrutiny resulted in the method of its metamorphosis being discovered.
