Page:Mars - Lowell.djvu/41

 be 1/3093500 of that of the Sun, or 10/94 of that of the Earth.

Knowing his mass, we know his average density, since to find it we have but to divide his mass by his volume. It proves to be 72/100 of that of the Earth. We also learn the force of gravity at his surface, inasmuch as this is directly as his mass and inversely as the square of his radius. It comes out 38/100 of that of the Earth. In consequence, all things there would weigh but 38/100 of their weight on earth; a man, for example, weighing 150 pounds here would weigh but 55 pounds if transported to the surface of Mars, and all manual labor would be lightened threefold.

So soon as the planet was scanned telescopically, he was seen to present a disk, round at times, at other times lacking somewhat of a perfect circle, showing like the Moon when two days off from full. Such appearance visibly demonstrated, first, that he was not a self-luminous body, and secondly, that he revolved about the Sun outside of the Earth. A glance at the diagram of the orbit will make time latter point clearer. If we draw a line from the Sun to the centre of Mars and pass a plane through the planet perpendicular to this line and to time plane of his orbit, this plane will divide the illumined half of him from the unillumined half. If now we draw another line from any point