304 LOWELL—THE CARTOUCHES OF MARS. |Dee. 4, 
zones I thought it best to take—those from the centre to 20° out on 
either side of it; next, those from 20° to 40° out; and last those 
from 40° to 60° away. ‘This tripartite arrangement had the advan- 
tage, which indeed was the reason of its adoption, of furnishing 
comparison between a marking’s visibility at different distances 
from the centre of the disk. And I may say in passing—for the 
came out in accordance with what realities on the planet’s surface 
would show. 
Were the disk always full the application would be simple and 
forthright. Being presented generally with a phase, certain cor- 
rections have first to be introduced. Since the illumination 
degrades from the point under the sun out to the terminator where 
it ceases altogether, a marking from this cause alone tends to 
disappear as it nears that boundary, and indeed within a certain 
distance of the night-line can never be seen at all. As such /erra 
non I took empirically a zone 25° in from the terminator, such 
being from my observations the mean value of the semi-obliterated — 
area. Subsequent calculation shows that this is about the value 
needed to equalize the chances of detection in the three pair of 
zones mentioned above when all the factors of position conducing 
to visibility are taken into account. 
Convenient epochs for testing the visibility of a canal were self- 
offered by its several presentations. A presentation of any part of 
the planet is the occasion of the presentment of that part to an ob- 
server upon the earth. As Mars takes forty minutes longer to 
rotate than our own globe, its longitudes lose on the average 9°.6 a 
day in coming to the disk’s meridian. In consequence of thus 
slowly falling behind time they complete an apparent backward 
revolution in about 38 days (from 37 to 41 days), since 9°.6 goes 
into 360° some thirty-eight times. After the lapse of this period, 
the two planets again show the same face to each other at the same 
hour. For a third of the time, therefore, the marking is well 
placed for observation ; for the other two-thirds, it is either not to 
be seen because the planet is below the horizon or practically invisible 
because the planet is not high enough up. Thus the presentations 
make natural epochs for comparing a marking with itself and 
noting any change in aspect it may have undergone in the interval. 
The data were furnished by the drawings. In the present inquiry 
these consisted of those made by me at the opposition of 1903 
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