one would build a good tall tower at Greenwich, straight
into the sky,--say a hundred miles into the sky,--of
course if you and I were east or west of it, and could
see it, we could tell how far east or west we were by
measuring the apparent height of the tower above our
horizon. If we could see so far, when the lantern with
a Drummond's light, "ever so bright," on the very top of
the tower, appeared to be on our horizon, we should know
we were eight hundred and seventy-three miles away from
it. The top of the tower would answer for us as the North
Star does when we are measuring the latitude. If we were
nearer, our horizon would make a longer angle with the
line from the top to our place of vision. If we were
farther away, we should need a higher tower.

But nobody will build any such tower at Greenwich, or
elsewhere on that meridian, or on any meridian. You see
that to be of use to the half the world nearest to it, it
would have to be so high that the diameter of the world
would seem nothing in proportion. And then, for the
other half of the world you would have to erect another
tower as high on the other side. It was this difficulty
that made Q. suggest the expedient of the Brick Moon.

For you see that if, by good luck, there were a ring
like Saturn's which stretched round the world, above
Greenwich and the meridian of Greenwich, and if it would
stay above Greenwich, turning with the world, any one
who wanted to measure his longitude or distance from
Greenwich would look out of window and see how high this