862 LOWELL—THE CARTOUCHES OF MARS. [Dec. 4, 
tor. Why the north tropic ones are more numerous than the south 
tropic in the list we shall see later. 
Taking now the position in time of the minimum value of the 
curve of each canal within a given zone, and then determining the 
mean minimum for all the canals in that zone, we find as follows: 
Mean Minimum, in 
days after the Or Exclusive of 
Summer Solstice. Starred Canals. 
Arctic Zone x... >... wiacebe Grob ect Sehueent ie fo) o* 
DL YATCHEIZONE koe). c\ers terns ieee a hee aioe 13 Lge 
North bemperateszone)s2 5.102) ero bee 22 22* 
Niarllgiincpit pZOney rive cone, Veg teins ae 34 3a7 
North Sub-lropie:Zone i. . farce ee cee 40 42* 
North Equatorial ,zone 23... s si. 4.20056 43 LM fis 
South Equatorial Zone, |... 06s ose ees oe 56 56* 
South! Dropicizone vs. acs tees ennce eee 68 68* 
South Sub-Fropic’ zone’, 2/005 sk. «aim osu. 95 95* 
Disclosed stands a steady progression in the time of minimum 
development of the canals as we travel from the neighborhood of 
the polar cap to the equator. The orderly advance becomes even 
more noticeable when certain canals which appear to contain mis- 
takes or misidentifications or mutual exchanges of visibility are 
eliminated. Such seem to be the Amenthes-Thoth-Nepenthes- 
Triton system, in which just after opposition the Thoth-Nepenthes- 
Triton apparently replaced the Amenthes, and then died down 
later as if nothing out of order had happened. The Indus and the 
Gihon II, or that part of the Gihon north of the Deuteronilus, are 
not impossibly another case of interchange. The two Sitacus 
and the Apis may be cases of straight li; masked in their earlier 
presentations by distance and unfavorable seeing. For the out-of- 
place development of the Isiacum, I am at a loss satisfactorily to 
account. Omitting the above canals from the count we get the 
second row of minima, which show a yet closer approach to uni- 
formity of progression. Indeed, if we now plot the mean curves or 
cartouches of the mean canals at ordinal intervals corresponding to 
the degrees of latitude at which they occur, we shall find that a 
straight line will nearly pass through all the points. This is shown 
in Plate XV, which, based on Mercator’s projection, makes of the 
straight line a curve slightly convex on the advancing side. But 
what is more remarkable, the progression does not stop at the 
Pa ow lela + 
ee ~ 
i a a eR Sa ae 
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