SCIENTIFIC USE OF TOE 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 



