to determine with accuracy. Fortunately this is matter chiefly of theoretic regret, as we now know the actual sizes to within a degree of exactness practically sufficient for most purposes but perturbations; to within about 1/300 part of the whole, so far as our ultimate measure is concerned, the distance we are off from the Sun.

A good idea of the method and some appreciation of the difficulty involved in it can be got by considering a precisely similar case, that of determining the distance of a spire a mile and three fifths away by shutting first one eye and then the other and noting the shift of the spire against its background. It is needless to add that without telescopic aid the determination is impossible, and that it is exceeding difficult with it.

Nevertheless, from the distance of the Sun determined in this manner, we find from measurements of the apparent disk of the planet made at Flagstaff that Mars is about 4,215 miles in diameter. This makes his surface a little more than a quarter that of the Earth and his volume about one seventh of hers.

The next point to find out is his mass, that is, the amount of matter he contains. This is very easy to determine when a planet has a satellite, and very difficult to determine when a planet has not. The reason is this: the mass of a body is known from the pull it exerts, inasmuch as this pull depends, by the law of gravitation, upon its mass and the square of its distance. If then we know the pull and the distance from which it is exerted, we can find the mass. Now we gauge the pull from its effects in causing some other body to move. By measuring, therefore, the motion of this other body, we learn the mass of the first one. To get this accurately the motion must be large enough to admit of satisfactory measurement in the first place, and be as uncomplicated with motions due to pulls of other bodies as possible, in the second. As each body pulls every other, and it is only their relative displacement we can measure, as we have no foothold in space, even the case of only two bodies presents difficulties of apportionment. We can learn the aggregate mass of the two, but not the separate mass of either alone unless it so happen that the mass of one is so insignificant compared with the other that the mass of that other may be taken as the mass of both. Now this is substantially realized in the case of the solar system. Owing to the greatly disproportionate size of primary and secondary bodies in it, the great size of the Sun as compared with that of any of the planets, and the great size of the planets as compared with their satellites (with the exception of the Moon, and she, fortunately, is an only child), the determination of the mass of the smaller by measurement of its motion about the larger,--as if only the pair of bodies under consideration existed, and the mass of both were concentrated in the greater of the two,-- is very nearly exact. Inconsequence each planet discloses with some accuracy the mass of the Sun, but tells next to nothing about its own mass; and in the same way each satellite reveals the mass of its primary. The Mass of a planet possessing a satellite is, therefore, easy of determination. Not so that of one which travels unattended. The only way to obtain its mass is from the perturbations or disturbing pulls it exerts upon the other planets, or upon stray comets from time to time, and these disturbances are, by the nature of the case, of a much smaller order of magnitude, to say nothing of the fact that all act coincidently to increased difficulty of disentanglement. The practical outcome of this in the case of Mars was that before his satellites were discovered the values obtained for his mass ranged all the way from 1/3,700,000 to 1/2,500,000, of the mass of the Sun, or, in other words, varied fifty per cent. His insignificant satellites, however, and just because they are insignificant, have made it possible to learn his mass with great exactness. It turns out to be 1/3,093,500 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 56 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