446 NEW OBSERVATIONS OF THE PLANET MERCUKY. 
tioiis. At tlie time the observations began the planet was between superior conjunc- 
tion and eastern elongation, and lay at the time very conveniently placed for 
comparison with Venus, the two planets being near each other, Mercury lying at 
first at a somewhat greater angular distance from the Sun, and then at a slightly less, 
Venus passing out and Mercury in on September 24. The difference in the albedo 
or intrinsic brightness of the surface of the two bodies was most marked. My initial 
note on Mercury, on August 21, reads: "Mercury, — strikingly not so bright in 
albedo as Venus ; just looked at on a brighter sky, too." At this time Mercury was 
nearer the Sun in the proportion of 100 to 165, and therefore his surface was 2.73 
times more brilliantly illuminated than hers. But the relative lustre of the two was 
even more disproportionate the other way. This relative appearance continued to be 
presented on every occasion, only with ever increasing contrast. For as Mercury's 
phase increased, his surface lost visibly in lustre ; that is, quite apart from the loss m 
area as a whole, what illuminated surface remained showed much less bright, square 
unit for square unit, than it had when the disk was nearer the full. A similar loss of 
albedo w^ith increase of phase is shown by our Moon. 
In consequence of this loss with increase of phase, the disproportion between Venus 
and Mercury is not quite so great as appeared on August 21, inasmuch as Venus at 
that time was the nearer to the full. 
On the other hand, I have compared Mercury with the Moon and found his surface 
the brighter of the two in about the proportion that his greater proximity to the Sun 
would cause. He thus shows as an airless body should. 
24. Explanation of the Map. — From the markings detected at Flagstaff and in 
Mexico is constructed the map of the planet in Plate XXVII. The face the planet 
shows us suggested the projection to be used, — parallel projection for the latitudes and 
an equidistant one for the longitudes. For since the axis of rotation is substantially 
perpendicular to the Earth's orbit, the latitudes are seen forever foreshortened in 
parallel projection, while the longitudes are all of them at one time or another seen 
as they are. 
To either side of the hemisphere of mean libration I have added wings on the 
same projection for such further longitudes as the libration reveals. 
The longitude of the centre of the disk when at its mean libration is taken as the 
zero meridian. 
25. Signification of the Symmetry of the Markings. — From all of the foregoing 
we see that Mercury is a dead body, like our own Moon. Was it always so or n 
the next question. With regard to this, the curious symmetry observed in 
th 
