ing found traces of atmosphere in some of these minor planets, but
it was generally thought he was mistaken. One reason we know so little
about this great swarm of minor planets is, that till recently none of
them showed a disk to the telescope. Inasmuch as only their light was
visible, they were indistinguishable from stars, except by their slow
motion. A hundred years ago only three hundred and fifty had been
discovered; our photographic star-charts have since then shown the
number recorded to exceed one thousand."

[Illustration: Comparative Size of Planets.]

CHAPTER IV.

PREPARING TO ALIGHT.

That afternoon Ayrault brought out some statistical tables he had
compiled from a great number of books, and also a diagram of the
comparative sizes of the planets. "I have been not a little puzzled at
the discrepancies between even the best authors," he said, "scarcely
any two being exactly alike, while every decade has seen accepted
theories radically changed." Saying which, he spread out the result of
his labours (shown on the following pages), which the three friends
then studied.

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(1) Mean distance from sun in millions of miles
(2) Semimajor axis of orbit, earth's distance as 1
(3) Eccentricity of orbit
(4) Planets inclination of orbit to elliptic
(5) Light at perihelion
(6) Light at apehelion
(7) Heat, earth as 1

(1) (2) (3) (4) (5) (6) (7)

Mercury... 36.0 0.387 0.2056 7@0'8" 10.58 4.59 6.67
Venus..... 67.2 0.723 0.0068 3@23'35" 1.94 1.91 1.91
The Earth. 92.9 1.000 0.068 0@0'0" 1.03 0.997 1.00
Mars......141.5 1.524 0.0933 1@51'2" 0.52 0.360 1.43
Asteroids 204.4 to 2.200 0.4 to 5@-35@ 325.2 to 3.500 0.34
Jupiter.. 483.3 5.203 0.0483 1@18'41" 0.04 0.034 0.037
Saturn... 886.0 9.539 0.0561 2@29'40" 0.012 0.0099 0.011
Uranus.. 1781.9 19.183 0.0463 0@46'20" 0.0027 0.0025 0.003
Neptune. 2791.6 30.055 0.0090 1@47'2" 0.0011 0.0011 0.001
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(1) MOVEMENT IN ORBIT. Velocity compared with earth as 1.
(2) MOVEMENT IN ORBIT. Period of revolution in years and days.
(3) MOVEMENT IN ORBIT. Orbital velocity in miles per second.
(4) Mean diameter in miles
(5)