Page:Popular Science Monthly Volume 14.djvu/307

Rh or little more than the greatest apparent diameter of Venus. We modestly lay claim to this small corner of the universe, denominated the solar system, and assert our right to possession by calling it ours. What is the area of this plane bounded by Neptune's orbit, with which the planes of the other planetary orbits nearly coincide? What is the space swept by the radius-vector of this planetary child of Adams's and Le Verrier's calculations? Since circles are as the squares of their radii, this area is 900 times that comprised within the earth's orbit. Breaking this unit up into smaller ones, we find it contains twenty-six billions, five hundred millions of millions of square miles; or, with reference to the earth's entire surface, the ratio between it and the area of its orbit is 13,520,000. But Neptune's orbit exceeds this 900 times!

Conceive this orbit immersed in the universal ether, like an immense ring mapped out on the surface of still water. A pebble dropped at the centre of this ring would send its widening wavelets outward with a perfectly definite velocity. So a wave of light, emanating from the sun, with a length of no more than the part of an inch, is propagated through this universal ether with such rapidity that in four hours and nine minutes it describes the entire area comprised within Neptune's path around the sun.

Across this vast interval quivers, too, in some mysterious way, that universal influence that we call gravitation. But at that outlying point, where Neptune holds on its silent course, it no longer exercises that dominant sway that characterizes it at the earth. The earth moves through 18.4 miles of its orbit every second, and is deflected from a straight line by the sun during that interval a little less than of an inch (0.11598 inch), Neptune travels 3.35 miles a second, and is deflected from a straight line during the same time only about  of an inch (0.000129 inch); yet by that slight pull the sun asserts its mastery, and brings Neptune round once in 164 years.

Vast as is this field of solar operations, we demand a still broader sphere for the exercise of our intellectual faculties. The successive eminences on which Astronomy has planted her appliances for more advanced operations stand related to each other as the terms, not of an arithmetical, but of a geometrical progression. Having settled the relative distances of this rather unsocial family of planets, the next advance is on to the stars. From the measurement of a base-line a few miles in extent, the astronomer essays, with undaunted hardihood, to fathom by triangulation the depths of space. The dog bays at the moon, the child stretches out its tiny hand to pluck the stars from the sky, and the astronomer applies his measuring-rod along the vibrating lines of light so far into immensity that blazing suns, exceeding in brightness the mid-day splendor of our own, dwindle to the luminous points of twinkling stars.

When Copernicus, the Polish astronomer, sought to extricate astronomy from the hopeless complexity into which it had become