The pendulum thus enables us to make a gravitational survey of the surface of the earth with the highest degree of accuracy. We cannot, however, infer that gravity alone affects the oscillations of the pendulum. We have seen how the earth rotates on its axis, and we have attributed the bulging of the earth at the equator to this influence. But the centrifugal force arising from the rotation has the effect of decreasing the apparent weight of bodies, and the change is greatest at the equator, and lessens gradually as we approach the poles. From this cause alone the attraction of the pendulum at the equator is less than elsewhere, and therefore the oscillations of the pendulum will take a longer time there than at other localities. A part of the apparent change in gravitation is accordingly due to the centrifugal force; but there is, in addition, a real alteration. In a work on astronomy it does not come within our scope to enter into further detail on the subject of our planet. The surface of the earth, its contour and its oceans, its mountain chains and its rivers, are for the physical geographer; while its rocks and their contents, its volcanoes and its earthquakes, are to be studied by the geologists and the physicists. CHAPTER X. MARS. Our nearer Neighbours in the Heavens--Surface of Mars can be Examined in the Telescope--Remarkable Orbit of Mars--Resemblance of Mars to a Star--Meaning of Opposition--The Eccentricity of the Orbit of Mars--Different Oppositions of Mars--Apparent Movements of the Planet--Effect of the Earth's Movement--Measurement of the Distance of Mars--Theoretical Investigation of the Sun's Distance--Drawings of the Planet--Is there Snow on Mars?--The Rotation of the Planet--Gravitation on Mars--Has Mars any Satellites?--Prof. Asaph Hall's great Discovery--The Revolutions of the Satellites--Deimos and Phobos--"Gulliver's Travels." The special relation in which we stand to one planet of our system has necessitated a somewhat different treatment of that globe from the treatment appropriate to the others. We discussed Mercury and Venus as distant objects known chiefly by telescopic research, and by calculations of which astronomical observations were the foundation. Our knowledge of the earth is of a different character, and attained in a different way. Yet it was necessary for symmetry that we should discuss the earth after the planet Venus, in order to give to the earth its true position in the solar system. But now that the earth has been passed in our outward progress from the sun, we come to the planet Mars; and here again we resume, though in a somewhat modified form, the methods that were appropriate to Venus and to Mercury.