This rate of flow, as indicated by the darkening arising from the growth of vegetation which follows the flow of the water down the canals, has been observed and noted many times at Flagstaff Observatory.
It was now perfectly clear to us why the "carets" are only seen in the particular places in which they have been observed by Professor Lowell and his colleagues. They are, in fact, only needed in connection with water-lifting apparatus, and locks and weirs, at the places where high-level canals connect with those at a lower level!
We were all very pleased at finding the solution of a problem which had been much discussed between us without arriving at any satisfactory conclusion.
John then asked Tellurio if he would be good enough to explain to us how it was that our observers on the earth saw some of the Martian canals doubled at some periods of the year and single at other times; and sometimes one of the twin canals was seen alone, and at other times the second one only was visible.
"It is a very simple matter, sir," replied Tellurio. "You will understand that we do not wish to waste any of our water, and as it is quite unnecessary to use all our canals at the same time, we only use those which are actually required. This arrangement also allows us to have a much greater depth of water in the canals than would be the case if they were all in use at once.
"Many of the canals are only required for irrigating seasonal crops; so as soon as the requisite amount of moisture has been acquired by the soil the water is turned from that canal into another one, passing through an area where a later seasonal crop is to be grown. This arrangement, moreover, applies not only to our double canals, but also to very many of the series which you have regarded as single canals."
Thus the mysteries connected with Mars were being cleared up one after the other; and having regard to the very simple and natural explanations we received, we could not help laughing as we talked the matter over and recalled the immense amount of discussion and wrangling which had occurred amongst our scientific men in connection with these matters, and especially at the difficulty they seemed to experience in believing that the canals could exist at all. Then there were those charges and theories of overstrained eyes, diplopia, and defective focussing, to say nothing of other suggestions. Well, I will not say any more upon this point.
In continuation of our discussion of the canal question, I asked Tellurio "Whether the canals and irrigation system had been the means of reclaiming any large areas of land which had previously been deserts?"
"Oh yes, sir," he answered, "that has been the case in many parts of our world; some very large areas indeed which were once deserts have now become very fertile. Quite apart from such reclamations, however, our canals and irrigation systems have also effectually checked the spread of desertism. If it had remained unchecked, probably by this time the entire surface of our planet would have become a desert."