(57) 2 ? r 



From their perviousness to stellar light, and other 

 considerations, Sir John Herschel drew some startling 

 conclusions regarding the density and weight of comets. 

 You know that these extraordinary and mysterious bod- 

 ies sometimes throw out tails 100,000,000 of miles in 

 length, and 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 

 this sphere with cometary matter, and make it our unit 

 of measure. An easy calculation informs us that 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 

 together, and suitably compressed, what do you suppose 

 its volume would be ? Sir John Herschel would prob- 

 ably 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, then, to sur- 

 round the earth at a hight above the surface which 

 would place it beyond the grosser matter that hangs in 

 the lower regions of the air say at the hight of the 

 Matterhorn or Mont Blanc. Outside this shell we have 

 the deep blue firmament. Let the atmospheric space 

 beyond t)ie 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 gentle- 

 man's portmanteau possibly his snuff-box might take it 



