

SCIENTIFIC USE OF THE IMAGINATION. 123 



matter, and make it our unit of measure. To produce a 

 comet's tail of the size just mentioned, about 300,000 

 such measures would have to be emptied into space. 

 Now suppose the whole of this stuff to be swept toge- 

 ther, 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 distance 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 should have the deep blue firma- 

 ment. Let the atmospheric space beyond the shell be 

 swept clean, and the sky-matter properly gathered up. 

 What would be its probable amount? 1 have some- 

 times thought that a lady's portmanteau would contain 

 it all. I have thought that even a gentleman's portman- 

 teau 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 con- 

 tinuity of its light. It is not in broken patches, nor 

 at scattered points, that the heavenly azure is revealed. 

 To the observer on the summit of Mont Blanc, the blue 

 is as uniform and coherent as if it formed the surface 

 of the most close-grained solid. A marble dome 



