XIX. On the remarkable appearances at the polar regions of the planet Mars, and its spheroidical figure; with a few hints relating to its real diameter and atmosphere.

Abstract

What I have to offer on the subject of the remarkable appearance at the polar regions of Mars, as well as what relates to the inclination of the axis, the position of the poles, and the spheroidical figure of that planet, is founded on a series of observations which I shall deliver in this paper; and after they have been given in the order they were made, it will be easy to shew, by a few deductions from them, that my theory of this planet is supported by facts which will sufficiently authorise the conclusions I have drawn from them. For the sake of better order and perspicuity, however, I shall treat each subject apart, and begin with the remarkable appearances about the polar regions. the observations on them were made with a view to the situation and inclination of the axis of Mars; for to determine these we cannot conveniently use the spots on its surface, in the manner which is practised on the sun The quantities to be measured are so small, and the observations of the center of Mars so precarious, and attended with such difficulties (since an error of only a few seconds would be fatal) that we must have recourse to other methods.