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378 which goes farthest from it. All the orbits are described in the standard, or west to east, direction. The most interesting characteristic in the distribution of the minor planets, first noted in 1866 by Daniel Kirkwood (1815-1895), is the existence of comparatively clear spaces in the regions where the disturbing action of Jupiter would by Lagrange's

principle (chapter, § 243) be most effective: for instance, at a distance from the sun about five-eighths that of Jupiter, a planet would by Kepler's law revolve exactly twice as fast as Jupiter; and accordingly there is a gap among the minor planets at about this distance.

Estimates of the sizes and masses of the minor planets are still very uncertain. The first direct measurements