during November. This becomes evident, numerically, on applying the correction for an atmosphere, which gives the following values:
Polar Diameters: October 15 to 23 inc. 9”.32 October 12 and 24 to 30 inc. 9”.31 November 2 to 21 inc. 9”.32 Equatorial Diameters: October 15 to 23 inc. 9”.37 October 12 and 24 to 30 inc. 9”.36 November 2 to 21 inc. 9”.37
The middle values are evidently somewhat too small, since they affect both the polar and equatorial diameters alike. Otherwise the variation in the values of the same diameter is less than the probable errors of observation. Taking the mean of all but the middle ones, we deduce the value for the polar flattening given above, 1/190 of the equatorial diameter.
From the correction for the effect of the atmosphere, we find the amount of the twilight arc upon the planet visible from the Earth to be about 10°. That of the Earth, as seen from the Earth’s surface, is 18°; but it is to be noticed that here the point of view is important. From the topmost layer of our air of sufficient density to be capable of reflecting light we are but forty miles away; from the corresponding layer of the Martian air we are forty millions of miles off. We cannot, therefore, expect to
