light and the other face dark, must be the actual state of things on Mercury.

There is one interesting qualification. In the case just imagined for the earth, supposing it to retain the present inclination of its axis while parting with its differential rotation, there would be an interchange of day and night once a year in the polar regions. On Mercury, whose axis appears to be perpendicular, a similar phenomenon, affecting not the polar regions but the eastern and western sides of the planet, is produced by the extraordinary eccentricity of its orbit. As the planet alternately approaches and recedes from the sun its orbital velocity, as we have already remarked, varies between the limits of twenty-three and thirty-five miles per second, being most rapid at the point nearest the sun. But this variation in the speed of its revolution about the sun does not, in any manner, affect the rate of rotation on its axis. The latter is perfectly uniform and just fast enough to complete one axial turn in the course of a single revolution about the sun. The accompanying figure may assist the explanation.

Diagram showing that, owing to the Eccentricity of its Orbit, and its Varying Velocity, Mercury, although making but One Turn on its Axis in the Course of a Revolution about the Sun, nevertheless experiences on Parts of its Surface the Alternation of Day and Night.

Let us start with Mercury in perihelion at the point A. The little cross on the planet stands exactly under the sun and in the center of the illuminated hemisphere. The large arrows show the direction in which the planet travels in its revolution about the sun, and the small curved arrows the direction in which it rotates on its axis. Now, in moving along its orbit from A to B the planet, partly because of its swifter motion when near the sun, and partly because of the elliptical nature of the orbit, traverses a greater angular interval with reference to the sun than the cross, moving with the uniform rotation of the planet on its axis, is able to traverse in the same time. As drawn in the diagram, the cross has moved through exactly ninety degrees, or one right angle, while the planet in its orbit has moved through considerably more than a right angle. In consequence of this gain of the angle of revolution upon the angle of rotation, the cross at B is no longer exactly under the sun, nor in the center of the illuminated hemisphere. It appears to have shifted its position toward the west, while the hemispherical cap of sunshine has slipped eastward over the globe of the planet.

In the next following section of the orbit the planet rotates through another right angle, but, owing to increased distance from the sun, the motion in the orbit now becomes slower until, when the planet arrives at aphelion, C, the angular difference disappears and the cross is once more just under the sun. On returning from aphelion to perihelion the same phenomena recur in reverse order and the line between day and night on the planet first shifts westward, attaining its limit in that respect at D, and then, at perihelion, returns to its original position.

Now, if we could stand on the sunward hemisphere of Mercury what, to our eyes, would be the effect of this shifting of the sun's position with regard to a fixed point on the planet's surface? Manifestly it would cause the sun to describe a great arc in the sky, swinging to and fro, in an east and west line, like a pendulum bob, the angular extent of the swing being a little more than forty-seven degrees, and the time required for the sun to pass from its extreme eastern to its extreme western position and back again being eighty-eight days. But, owing to the eccentricity of the orbit, the sun swings much faster toward the east than toward the west, the eastward motion occupying about thirty-seven days and the westward motion about fifty-one days.

The Regions of Perpetual Day, Perpetual Night, and Alternate Day and Night on Mercury. In the Left-Hand View the Observer looks at the Planet in the Plane of its Equator; in the Right-Hand View he looks down on its North Pole..

Another effect of the libratory motion of the sun as seen from Mercury is represented in the next figure, where we have a view of the planet showing both the day and the night hemisphere, and where we see that between the two there is a region upon which the sun rises and sets once every eighty-eight days. There are, in reality, two of these lune-shaped regions, one at the east and the other at the west, each between 1,200 and 1,300 miles broad at the equator. At the sunward edge of these regions, once in eighty-eight days, or once in a Mercurial year, the sun rises to an elevation of forty-seven degrees, and then descends again straight to the horizon from which it rose; at the nightward edge, once in eighty-eight days, the sun peeps above the horizon and quickly sinks from sight again. The result is that, neglecting the effects of atmospheric refraction, which would tend to expand the borders of the domain of sunlight, about one quarter of the entire surface of Mercury is, with regard to day and night, in a condition resembling that of our polar regions, where there is but one day and one night in the course of a year—and on Mercury a year is eighty-eight days. One half of the remaining three quarters of the planet's surface is bathed in perpetual sunshine and the other half is a region of eternal night.

And now again, what of life in such a world as that? On the night side, where no sunshine ever penetrates, the temperature must be extremely low, hardly greater than the fearful cold of open space, unless modifying influences beyond our ken exist. It is certain that if life flourishes there, it must be in such forms as can endure continual darkness and excessive cold. Some heat would be carried around by atmospheric circulation from the sunward side, but not enough, it would seem, to keep water from being perpetually frozen, or the ground from being baked with unrelaxing frost. It is for the imagination to picture underground dwellings, artificial sources of heat, and living forms suited to unearthlike environment.

What would be the mental effects of perpetual night upon a race of