direction in which thou wilt launch each wailing world; weigh well the mighty impulse soon to be given, for out of the myriads of directions, and the myriads of impulsive forces, there comes but a single combination that will secure the perpetuity of your complex scheme.  In vain does the bewildered finite spirit attempt to fathom this mighty depth.  In vain does it seek to resolve the stupendous problem.  It turns away, and while endued with omnipotent power, exclaims, ’Give to me infinite wisdom, or relieve me from the impossible task!’"-0.  M. MITCHEL, LL.  D.

[Page 99] VI.

THE PLANETS, AS SEEN FROM SPACE

If we were to go out into space a few millions of miles from either pole of the sun, and were endowed with wonderful keenness of vision, we should perceive certain facts, viz:  That space is frightfully dark except when we look directly at some luminous body.  There is no air to bend the light out of its course, no clouds or other objects to reflect it in a thousand directions.  Every star is a brilliant point, even in perpetual sunshine.  The cold is frightful beyond the endurance of our bodies.  There is no sound of voice in the absence of air, and conversation by means of vocal organs being impossible, it must be carried on by means of mind communication.  We see below an unrevolving point on the sun that marks its pole.  Ranged round in order are the various planets, each with its axis pointing in very nearly the same direction.  All planets, except possibly Venus, and all moons except those of Uranus and Neptune, present their equators to the sun.  The direction of orbital and axial revolution seen from above the North Pole would be opposite to that of the hands of a watch.

[Illustration:  Fig. 38.—­Orbits and Comparative Sizes of the Planets.]

The speed of this orbital revolution must be proportioned to the distance from the sun.  The attraction of the sun varies inversely as the square of the distance. [Page 100] It holds a planet with a certain power; one twice as far off, with one-fourth that power.  This attraction must be counterbalanced by centrifugal force; great force from great speed when attraction is great, and small from less [Page 101] speed when attractive power is diminished by distance.  Hence Mercury must go 29.5 miles per second—­seventy times as fast as a rifle-ball that goes two-fifths of a mile in a second—­or be drawn into the sun; while Neptune, seventy-five times as far off, and hence attracted only 1/5626 as much, must be slowed down to 3.4 miles a second to prevent its flying away from the feebler attraction of the sun.  The orbital velocity of the various planets in miles per second is as follows: 

Mercury  29.55  |  Jupiter  8.06
Venus    21.61  |  Saturn   5.95
Earth    18.38  |  Uranus   4.20
Mars     14.99  |  Neptune  3.36

Hence, while the earth makes one revolution in its year, Mercury has made over four revolutions, or passed through four years; the slower Neptune has made only 1/164 of one revolution.