fancying that it foretold their success. The history of the
Norman Conquest was worked in tapestry--that is to say, in what we
should call crewels on a strip of linen--and in this record the comet
duly appears. Look at him in the picture as the Normans fancied him. He
has a red head with blue flames starting from it, and several tails. The
little group of men on the left are pointing and chattering about him.
We can judge what an impression this comet must have made to be recorded
in such an important piece of work.

[Illustration: THE COMET IN THE BAYEUX TAPESTRY.]

But we are getting on too fast. We have yet to learn how anyone can know
that the comet which appeared at the time of the Norman Conquest is the
same as that which has come back again at different times, and above
all, how anyone can tell that it will come again in the year 1910. All
this involves a long story.

Before the invention of telescopes of course only those comets could be
seen which were of great size and fine appearance. In those days men
did not realize that our world was but one of a number and of no great
importance except to ourselves, and they always took these blazing
appearances in the heavens as a particular warning to the human race.
But when astronomers, by the aid of the telescope, found that for one
comet seen by the eye there were hundreds which no mortal eye unaided
could see, this idea seemed, to say the least of it, unlikely. Yet even
then comets were looked upon as capricious visitors from outer space;
odd creatures drawn into our system by the attraction of the sun, who
disappeared, never to return. It was Newton, the same genius who
disclosed to us the laws of gravity, who first declared that comets
moved in orbits, only that these orbits were far more erratic than any
of those followed by the planets.

So far we have supposed that the planets were all on what we should call
a level--that is to say, we have regarded them as if they were floating
in a sea of water around the sun; but this is only approximately
correct, for the orbits of the planets are not all at one level. If you
had a number of slender hoops or rings to represent the planetary
orbits, you would have to tilt one a little this way and another a
little that way, only never so far but that a line through the centre
of the hoop from one side to another could pass through the sun. The way
in which the planetary orbits are tilted is slight in comparison with
tha