ho invented the telescope,
turned it on to the sky in 1610, when our King Charles I. was on the
throne, and he saw these curious bodies which at first he could not
believe to be moons. The four which he saw vary in size from two
thousand one hundred miles in diameter to nearly three thousand six
hundred. You remember our own moon is two thousand miles across, so even
the smallest is larger than she. They go round at about the same level
as the planet's Equator, and therefore they cross right in front of him,
and go behind him once in every revolution. Since then the other three
have been discovered in the band of Jupiter's satellites--one a small
moon closer to him than any of the first set, and two others further
out. It was by observation of the first four, however, that very
interesting results were obtained. Mathematicians calculated the time
that these satellites ought to disappear behind Jupiter and reappear
again, but they found that this did not happen exactly at the time
predicted; sometimes the moons disappeared sooner than they should have
done, and sometimes later. Then this was discovered to have some
relation to the distance of our earth from Jupiter. When he was at the
far side of his immense orbit he was much more distant from us than when
he was on the nearer side--in fact, the difference may amount to more
than three hundred millions of miles. And it occurred to some clever man
that the irregularities in time we noticed in the eclipses of the
satellites corresponded with the distance of Jupiter from us. The
further he drew away from us, the later were the eclipses, and as he
came nearer they grew earlier. By a brilliant inspiration, this was
attributed to the time light took to travel from them to us, and this
was the first time anyone had been able to measure the velocity or speed
of light. For all practical purposes, on the earth's surface we hold
light to be instantaneous, and well we may, for light could travel more
than eight times round the world in one second. It makes one's brain
reel to think of such a thing. Then think how far Jupiter must be away
from us at the furthest, when you hear that sometimes these eclipses
were delayed seventeen minutes--minutes, not seconds--because it took
that time for light to cross the gulf to us!

[Illustration: JUPITER AND HIS PRINCIPAL MOONS.]

Sound is very slow compared with light, and that is why, if you watch a
man hammering at a distance, the stroke he