ed by Mars on the day in question. From this result, combined with
the modern meridian observations, we learn that the displacement of Mars
by the attraction of the earth has, in the lapse of two centuries, grown
to about five minutes of arc (294 seconds). It has been maintained that
this cannot be erroneous to the extent of more than a second, and hence
it would follow that the earth's mass is determined to about one
three-hundredth part of its amount. If no other error were present, this
would give the sun's distance to about one nine-hundredth part.

[Illustration: Fig. 50.--Relative Sizes of Mars and the Earth.]

Notwithstanding the intrinsic beauty of this method, and the very high
auspices under which it has been introduced, it is, we think, at present
hardly worthy of reliance in comparison with some of the other methods.
As the displacement of Mars, due to the perturbing influence of the
earth, goes on increasing continually, it will ultimately attain
sufficient magnitude to give a very exact value of the earth's mass, and
then this method will give us the distance of the sun with great
precision. But interesting and beautiful though this method may be, we
must as yet rather regard it as a striking confirmation of the law of
gravitation than as affording an accurate means of measuring the sun's
distance.

[Illustration: Fig. 51.--Drawing of Mars (July 30th, 1894).]

[Illustration: Fig. 52.--Drawing of Mars (August 16th, 1894).]

[Illustration: Fig. 53.--Elevations and Depressions on the "Terminator"
of Mars (August 24th, 1894).]

[Illustration: Fig. 54.--The Southern Polar Cap on Mars (July 1, 1894).]

The close approaches of Mars to the earth afford us opportunities for
making a careful telescopic scrutiny of his surface. It must not be
expected that the details on Mars could be inspected with the same
minuteness as those on the moon. Even under the most favourable
circumstances, Mars is still more than a hundred times as far as the
moon, and, therefore, the features of the planet have to be at least one
hundred times as large if they are to be seen as distinctly as the
features on the moon. Mars is much smaller than the earth. The diameter
of the planet is 4,200 miles, but little more than half that of the
earth. Fig. 50 shows the comparative sizes of the two bodies. We here
reproduce two of the remarkable drawings[16] of Mars made by Professor
William H. Pickering at the Lowell Observatory, Flagstaff A.