858 LOWELL—THE CARTUUCHES OF MARS. [Dec. 4, 
take the latitude as a probable criterion, we shall suddenly be 
aware of a very different state of things. As the canals are not 
points but lines, we must select for purposes of precision some 
point in them as their distinctive latitude and longitude. Their 
mean point, or more properly the mean of all their points, has 
therefore been taken in each case, since it is with mean values that 
we find ourselves concerned. On this principle we may classify 
the canals by zones of latitude, advancing down the disk from the 
north polar cap. The canals were therefore ticketed and arranged 
according to the following zones: 
Arctic zone, containing the canals whose mean latitude lay between 86°N.-65°N. 
Sub-Arctic zone, « 6“ 6 “ 65°N.—50°N, 
North Temperate zone, “ “ “ “ 50°N.-35°N. 
North Sub-Tropic zone, “ " +  —-35°N.—25°N: 
North Tropic zone, & és 6 “ 25°N.—I0°N. 
North Equatorial zone, <“ ss sc ce 10°N,—0° 
South Equatorial zone, “ “ of x 0° =-10°S. 
South Tropic zone, ss “ ss “ 10°S.—25°S, 
South Sub-Tropic zone, “ “ «6 “ 25°S.-35°S. 
86°N. was taken as starting-point because of the coming down of 
the north polar cap to about this latitude throughout the course of 
the observations. On the other hand the lowest zone extends only 
to 35°S., because, owing to the tilt of the north pole of the planet 
toward the earth, a tilt which ranged between 21°.1 and 25°.9 dur- 
ing the same period, the farthest observable canal south had 27°S. 
for its mid-point. The date at which each canal was at its mini- 
mum visibility is shown in the following list : 
Time OF MINIMUM DEVELOPMENT OF CANALS. 
Arctic Canals—86°-65° Lat. North. 
Lat. No. Days After 
N. Name. of Canals. Summer Solstice. 
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