Joty—On the Origin of the Canals of Mars. 263 
This explanation leaves the supposition that vegetation is concerned in the 
variable appearance of the canals as one which, although not necessary, is still not 
improbable. I may observe here that the argument for the presence of vege- 
tation from the observation of bluish-green tints in the darker areas requires 
to be very carefully received, seeing that just such bluish-green tints would appear, 
as a physiological optical effect when dark areas on a brick-red background 
were under observation. If this red colour is deducted from our range of visible 
wave-lengths, the residium would give just such a bluish-green as Mr. Lowell 
depicts in his ‘‘ Mars.” Such residual sensation is that which the eye will per- 
ceive looking from the brighter red to the dark areas. This, however, will not 
explain the changes of tints observed by Lowell. Without having recourse to 
double or triple canals, we find, then, a reason for the varying conspicuousness of 
the lines in the natural conveyance of water by diffusion through the atmosphere, 
and its precipitation more abundantly asthe supply of water increases with the 
advancing year. Slight atmospheric currents prevailing at particular seasons, as in 
the case of terrestrial atmospheric currents, will be sufficient reason for the erratic 
nature of the effects observed. To this may be added effects due to differences in 
the elevations of the low mountain ranges. 
On the Curvature of the Lines. 
It remains now to enter on the question of the amount of support which the 
foregoing views receive from an examination of the actual course of the lines as 
observed by Professor Schiaparelli and by Mr. Lowell and his colleagues. 
If the foregoing views have any foundation in fact, the curvature of the lines, 
where this is sufficiently definite to admit of investigation, must be such as the 
composition of the motions of a uniformly rotating sphere and a point rotating 
uniformly round the centre of this sphere, will give rise to. 
Certain conditions attending the composition of these motions are, in the first 
place, most probably prohibited from entering into the circumstances of the case 
under consideration. Mars’ present rate of axial rotation cannot well be supposed 
to have been at any time /ess than its present rate. This excludes the great circle 
from being among the curves, for this curve, of course, corresponds to the particu- 
lar case when the planet is at rest upon its axis, and the satellite rotating round it. 
If Mr. Lowell’s contention that the curves mapped by him are indeed great circles 
is true, then the present hypothesis is hardly tenable. A very brief investigation 
suffices to show that the curves on Mr. Lowell’s map where these curves are at all 
sufficiently prolonged to enable us to carry them, by safe exterpolation, to the 
equator, are not great circles. In the case of the great circle, the points of inter- 
section with the equator must span 180°. This is, a¢ a glance, evidently 
