n employed by writers, is borrowed from the rate at
which an express train travels.

Let us imagine, for instance, that we possess an express train which is
capable of running anywhere, never stops, never requires fuel, and
always goes along at sixty miles an hour. Suppose we commence by
employing it to gauge the size of our own planet, the earth. Let us send
it on a trip around the equator, the span of which is about 24,000
miles. At its sixty-miles-an-hour rate of going, this journey will take
nearly 17 days. Next let us send it from the earth to the moon. This
distance, 240,000 miles, being ten times as great as the last, will of
course take ten times as long to cover, namely, 170 days; that is to
say, nearly half a year. Again, let us send it still further afield, to
the sun, for example. Here, however, it enters upon a journey which is
not to be measured in thousands of miles, as the others were, but in
millions. The distance from the earth to the sun, as we have seen in the
foregoing table, is about 93 millions of miles. Our express train would
take about 178 _years_ to traverse this.

Having arrived at the sun, let us suppose that our train makes a tour
right round it. This will take more than five years.

Supposing, finally, that our train were started from the sun, and made
to run straight out to the known boundaries of the solar system, that is
to say, as far as the orbit of Neptune, it would take over 5000 years to
traverse the distance.

That sixty miles an hour is a very great speed any one, I think, will
admit who has stood upon the platform of a country station while one of
the great mail trains has dashed past. But are not the immensities of
space appalling to contemplate, when one realises that a body moving
incessantly at such a rate would take so long as 10,000 years to
traverse merely the breadth of our solar system? Ten thousand years!
Just try to conceive it. Why, it is only a little more than half that
time since the Pyramids were built, and they mark for us the Dawn of
History. And since then half-a-dozen mighty empires have come and gone!

Having thus concluded our general survey of the appearance and
dimensions of the solar system, let us next inquire into its position
and size in relation to what we call the Universe.

A mere glance at the night sky, when it is free from clouds, shows us
that in every direction there are stars; and this holds good, no matter
what portion of the globe we