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SCIENTIFIC USE OF THE IMAGINATION. 149

50,000 miles in diameter. The diameter of our earth is
8,000 miles. Both it and the sky, and a good portion of
space beyond the sky, would certainly be included in a
sphere 10,000 miles across. Let us fill a hollow sphere of
this diameter with cometary matter, and make it our unit
of measure. To produce a comet’s tail of the size just men-
tioned, about 300,000 such measures would have to be
emptied into space. Now, suppose the whole of this stuff
to be swept together and suitably compressed, what do you
suppose its volume would be ? Sir John Herschel would
probably tell you that the whole mass might be carted
away at a single effort by one of your dray-horses. In fact,
I do not know that he would require more than a small
fraction of a horse-power to remove the cometary dust.
After this you will hardly regard as monstrous a notion I
have sometimes entertained concerning the quantity of
matter in our sky. Suppose a shell to surround the earth
at a height above the surface which would place it beyond
the grosser matter that hangs in the lower regions of the
air—say at the height of the Matterhorn or Mont Blanc.
Outside this shell we have the deep-blue firmament. Let
the atmospheric space beyond the shell be swept clean, and
let the sky-matter be properly gathered up. What is its
probable amount ? I have sometimes thought that a lady’s
portmanteau would contain it all. I have thought that
even a gentleman’s portmanteau—possibly his snuff-box—
might take it in. And whether the actual sky be capable
of this amount of condensation or not, I entertain no doubt
that a sky quite as vast as ours, and as good in appearance,
could be formed from a quantity of matter which might be
held in the hollow of the hand.

Small in mass, the vastness in point of number of the
particles of our sky may be inferred from the continuity of
its light. It is not in broken patches, nor at scattered points
that the heavenly azure is revealed. To the observer on

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