ble, while to the unaided eye they are
almost all invisible.

In the diagram (p. 234) of the orbits of the various planets, it is
shown that a wide space exists between the orbit of Mars and that of
Jupiter. It was often surmised that this ample region must be tenanted
by some other planet. The presumption became much stronger when a
remarkable law was discovered which exhibited, with considerable
accuracy, the relative distances of the great planets of our system.
Take the series of numbers, 0, 3, 6, 12, 24, 48, 96, whereof each number
(except the second) is double of the number which precedes it. If we now
add four to each, we have the series 4, 7, 10, 16, 28, 52, 100. With the
exception of the fifth of these numbers (28), they are all sensibly
proportional to the distances of the various planets from the sun. In
fact, the distances are as follows:--Mercury, 3.9; Venus, 7.2; Earth,
10; Mars, 15.2; Jupiter, 52.9; Saturn, 95.4. Although we have no
physical reason to offer why this law--generally known as Bode's--should
be true, yet the fact that it is so nearly true in the case of all the
known planets tempts us to ask whether there may not also be a planet
revolving around the sun at the distance represented by 28.

So strongly was this felt at the end of the eighteenth century that some
energetic astronomers decided to make a united effort to search for the
unknown planet. It seemed certain that the planet could not be a large
one, as otherwise it must have been found long ago. If it should exist,
then means were required for discriminating between the planet and the
hosts of stars strewn along its path.

The search for the small planet was soon rewarded by a success which has
rendered the evening of the first day in the nineteenth century
memorable in astronomy. It was in the pure skies of Palermo that the
observatory was situated where the memorable discovery of the first
known minor planet was made by Piazzi. This laborious and accomplished
astronomer had organised an ingenious system of exploring the heavens
which was eminently calculated to discriminate a planet among the starry
host. On a certain night he would select a series of stars to the number
of fifty, more or less, according to circumstances. With his meridian
circle he determined the places of the chosen objects. The following
night, or, at all events, as soon as convenient, he re-observed the
whole fifty stars with the same instrument and in the same