It may seem to the reader as if what was seen in 1903 was but an unimportant advance over the observed phenomena of 1896-1897. Not so, however. For with the earlier instances it was not possible to be sure of the precise limits of the spot with regard to the double. The Coloe Palus, on the one hand, did not fill all the space apparently between the double Phison; while the Lucus Ismenius more than did so with the double Euphrates. To have set down the different appearances to insufficient definition would have been a great mistake, as subsequent observation has served to show. The Lucus Ismenius instances this. In 1896-1897 it was seen terminating the Euphrates, blocking all the space between the two lines and extending a little upon either side of them. Now, from its appearance in 1901 it was evident that the effect had been produced by twin spots lying along the Deuteronilus, the axis joining them being perpendicular to the Euphrates. In 1903 the relation was still better explained by what appeared then, when not only did the two spots stand out, but the Euphrates showed with a line running centrally into each.
Although originally seen by Schiaparelli as a single spot and so at first seen by me, better acquaintance with the disk disclosed to both observers its really dual character. As this pair has persisted through all three of the most recent oppositions, it seems fairly certain that it is always of this character, and more fitting, therefore, to give it the plural appellative. This is the single instance of a double oasis. There are many that lie close together and might be taken as such; but this is the only one where the connection is intrinsic. According to measures of the drawings of 1905 extending through six presentations, the distance between the twin oases is 4°.2.
Their relation to the canals which run into them is of the most complicated description and of the most suggestive character. For to the twin spots converge no less than seven double canals, one wedge-shaped pair and three single canals, a most goodly number of communication lines. Four of the double canals run into the oases with one line to each; these canals are the Astaboras, the Naarmalcha, the Euphrates, and the Hiddekel. Three doubles, the Protonilus, the Djihoun, and the Deuteronilus, embrace the oases between their two lines, while, in the singles, the canal connects with one or other of the twins, as the case may be.
Now, there is method as to which of the doubles shall straddle, which embrace, the two Ismenii. Those which leave the place parallel or nearly so to the direction joining the Luci, inclose them both; those of which the setting forth is at an angle to this direction depart, each line of the pair, from the eastern and the western spot respectively.
Peculiar association of the Luci Ismenii with double canals.
Consider, now, the disposition of these seven pairs of lines. All of them lie in one semicircle about the Luci, beginning with the Protonilus on the east and ending with the Deuteronilus on the west. Furthermore, all follow approximately arcs of great circles, except the Djihoun, and all send one of their twin lines to one Lucus, one to the other. The data are enough to make this statement possible. Although the west line of the Naarmalcha has not been caught entering its oasis, the east one has been seen to enter the other, and the width of the double shows that the west one must enter the corresponding spot. In the case of the Astaboras the double has only been observed as far as the Vexillum, but the south line has continued on to the west Ismenius, and here again the width makes it certain that were the canal double throughout, the other line must enter the east Ismenius. From the base line of the Proto-Deuteronilus the inclinations of the seven pairs are as follows:—
| Protonilus | 0° | Due East |
| Astaboras | 40° | North of East |
| Naarmalcha | 70° | North of East |
| Euphrates | 80° | North of West |
| Hiddekel | 55° | North of West |
| Djihoun | 0° | North of West |
| Deuteronilus | 0° | Due West |
Now, the width between the two lines of the four canals to the east increases regularly from the Protonilus round; the Protonilus being the narrowest double, the Astaboras the next, the Naarmalcha the next, and the Euphrates the widest. And from the width between the twin oases, it would seem that they severally enter the centres of them. What takes place in the case of the Hiddekel, which is wider than its tilt would imply, and in the Djihoun, which is narrower, is not so clear. But that they enter the oases in some place is certain.
