The 109 canals included all the more conspicuous canals on the planet at that opposition, all those that lent themselves by the sufficient frequency with which they were seen to a statistical result. They lay spread all the way between the edge of the polar cap in latitude 87° north to the extreme limit south, at which the then tilt of the north pole toward the earth permitted of canal recognition. This southern limit was in about latitude 35° south. Farther south than this vision became too oblique, amounting as it did, with an adverse tilt of twenty-five degrees to start with, to something over sixty degrees, for detection of such fine markings to be possible. Between the two limits thus imposed, by the perpetual snow on the one side and the observational tilt on the other, the 109 canals were distributed by zones as follows:— =================+===============+=========== ZONE | LATITUDE | NUMBER OF | | CANALS -----------------+---------------+----------- North Polar | 87° N.-78° N. | 1 Arctic | 78° N.-66° N. | 9 Sub-Arctic | 66° N.-51° N. | 9 North Temperate | 51° N.-37° N. | 11 North Sub-Tropic | 37° N.-24° N. | 18 North Tropic | 24° N.-12° N. | 21 North Equatorial | 12° N.- 0° N. | 14 South Equatorial | 0° N.-12° S. | 17 South Tropic | 12° S.-24° S. | 7 South Sub-Tropic | 24° S.-37° S. | 2 =================+===============+=========== As the latitude of a canal in the investigation was taken as that of its mid-point, such being the mean value of its successive parts, the latitudes about which information was obtained lay within the limits given above, the most northern canal, the Jaxartes N having for its mid-latitude 78° north, and the most southern, the Nectar, that of 27° south. The zones comprised each a belt of territory about thirteen degrees wide, the first being less solely because in part occupied by the permanent polar cap.