the Sinus Titanum when compared with those about the Solis Lacus. In August the former were but faintly visible; in November they had become evident; and yet, during this interval, little change in conspicuousness had taken place in the canals in the Solis Lacus region.
With like disregard of the effect due to distance, the canals to the east of the Ganges showed better at the November presentation of that region than they had at the October one, although the planet was actually farther off at the later date, in the proportion of 21 to 18.
A more striking instance of the irrelevancy of distance in the matter was observed in the same region by Schiaparelli in 1877. It is additionally interesting as practically dating his discovery of the canals. In early October of that year, on the evenings of the 2d and the 4th, he tells us, under excellent definition, and with the diameter of the planet's disk 21" of arc, the continental region between the Pearl-Bearing Gulf and the Bay of the Dawn was quite uniformly, nakedly bright, and destitute
- A presentation of any part of the planet is the occasion when that part of the disk is turned toward the observer. Many causes combine to make the face presented each night vary, but the chief one is that the Earth rotates about forty-one minutes faster than Mars, and consequently gains a little less than ten degrees on him daily. After about thirty-seven days, therefore, the two planets again present the same face to each other at the same hour.