| Double Canals of Mars arranged according to Latitude | At Opposition of 1903 Alone | At All Oppositions so far observed at Flagstaff | |
| Between 30° S. and 20° S. | Tithonius, Nectar, Laestrygon | 2 | 3 |
| Between 20° S. and 10° S. | Jamuna, Ganges, Gigas, Laestrygon, Cyclops, Titan, Tartarus, Polyphemus, Tithonius | 7 | 9 |
| Between 10° S. and 0° | Jamuna, Ganges, Gigas, Laestrygon, Cyclops,Cerberus S, Aethiops, Lethes, Amenthes, Triton, Phison, Euphrates, Titan, Tartarus, Adamas, Typhon, Vexillum, Asopus, Naarmalcha, Polyphemus | 15 | 20 |
| Between 0° and 10° N. | Gihon, Djihoun, Jamuna, Ganges, Gigas, Laestrygon, Cerberus N, Cyclops, Cerberus S, Eunostos, Aethiops, Lethes, Amenthes, Triton, Nepenthes, Phison, Euphrates, Sitacus, Hiddekel, Tartarus, Adamas, Asopus, Typhon, Vexillum, Cocytus, Is, Avernus N, Naarmalcha, Polyphemus | 21 | 29 |
| Between 10° N. and 20° N. | Gihon, Djihoun, Jamuna, Nilokeras I and II[[4]], Nilokeras I, Ganges, Gigas, Eunostos, Aethiops, Lethes, Amenthes, Thoth, Astaboras, Phison, Sitacus, Euphrates, Hiddekel, Adamas, Asopus, Gelbes, Avernus N, Erebus, Naarmalcha, Vexillum, Is, Dis | 18 | 26 |
| Between 20° N. and 30° N. | Gihon, Djihoun, Jamuna, Nilokeras I & II,[[4]] Nilokeras I, Alander, Hyblaeus, Lethes, Amenthes, Thoth, Sitacus, Astaboras, Vexillum, Phison, Euphrates, Hiddekel, Adamas, Eunostos, Aethiops, Gelbes, Avernus N, Naarmalcha, Is | 17 | 23 |
| Between 30° N. and 40° N. | Deuteronilus, Alander, Nar, Marsias, Fretum Anian, Amenthes, Thoth, Vexillum, Phison, Euphrates, Hiddekel, Adamas, Eunostos, Djihoun, Gihon, Nilokeras I, Chaos, Gelbes, Aethiops, Naarmalcha | 12 | 20 |
| Between 40° N. and 50° N. | Fretum Anian, Pyramus, Protonilus, Propontis[[4]] | 3 | 4 |
| Between 50° N. and 60°&nbpp;N. | Callirrhoe, Fretum Anian, Pierius | 3 | 3 |
| Between 60° N. and 63° N. | Pierius, Callirrhoe | 2 | 2 |
From this tabulating of them it is apparent that the doubles are practically confined to the zones within forty degrees of the equator. Only 7% of them straggle farther north than this, while above 63° north latitude and 35° south latitude there are none. Such a distribution is not in proportion to the areas of the zones, which though diminishing toward the poles do so at no such rate. The surface included between the equator and 40° of latitude is 65% of the hemisphere, whereas the fraction of the number of doubles found there is 93%. The doubles are, then, an equatorial feature of the planet, confined to the tropic and temperate belts.
To perceive the tropical character of the doubles in another way we have but to consider the zonal distribution of the single canals. Unlike the former the latter do not thin out as one advances toward the poles; since in the arctic regions single canals bemesh the surface as meticulously as elsewhere. It is only that they there replace the doubles; or, not to put the cart before the horse, it is the doubles that in part replace the singles in the tropics. And that this arrangement has something physical behind it by way of cause is curiously shown by two canals, the Arnon and the Kison, which are neither of the one kind nor yet the other, but a cross between the two. For the Arnon and the Kison are convergent doubles; the two lines of the Kison leaving a common point at the edge of the polar cap and separating as they travel south, while the two Arnon take up and continue the divergence, connecting at last with the parallel pair of the Euphrates. These canals thus make transition between the true doubles and the true singles, and may be looked upon as endowed with the potentialities of both. From their association with the double Euphrates, it is clear that the transition between the two forms is not only formal but physical, and that the stopping of the dual condition at the fortieth parallel is not the outcome of chance.
It may occur to the thoughtful that the doubles appear confined to the more tropical portions of the planet because of a better presentation of those zones, the reader supposing the planet to be seen axised perpendicularly to the plane of sight, as geographies represent the earth’s globe. The supposition, however, is erroneous. We sometimes see the planet so, but more often not. Such is the tilt of the Martian axis to the plane of the Martian ecliptic that the different zones are rarely seen on an even keel, so to speak, their aspects shifting totally from one opposition to another. What shows in mid-disk on one occasion may be forty-eight degrees removed from it at another, a distance amounting to three-quarters of the way from apparent equator to apparent pole.
Thus the double canals are for some intrinsic reason equatorial features of the planet as opposed to polar ones. And this not simply because of greater space there. Duality is a result of conditions intrinsic to the several localities. What the cause may be is related to the character of the things themselves, which we shall later consider. For the moment we may note that the fact disposes quietly of the diplopic theory of their manufacture. For, for diplopic doubles to show such respect for the equator would betoken a courtesy in them to be commended of Sydney Smith.
But this is not their only geographic bias. In addition to not being partial to the poles, the double canals show a certain exclusiveness toward the dark areas generally. Not only do they avoid the arctic and antarctic zones entirely, but they largely shun the blue-green regions. In these but two suspicions of doubles occur, in the Aonium Sinus, although single canals there are as numerous as anywhere else on the planet.
Nevertheless, although they avoid running through them, they run from them in a manner that is marked. Proceeding from the great diaphragm are no less than 28 out of the 53 doubles. Connecting directly with these are 17 more; while the remaining 8 are also associated through the intermediarism of dark areas, the Solis Lacus and the Trivium.
In like relation to dark regions, they are limited on the north by the Mare Acidalium, the Propontis, the Wedge of Casius and their interconnecting bands, the Pierius, Callirrhoe, Helicon. In this manner do they form a broad girdle round the planet’s waist, leaving the polar extremities bare.
| [4] | Very wide and possibly not of the same class. |