148 Things not generally Known. 



with a large proportion of white. As the moon is yellow, the 

 blue of the air assumes somewhat of a greenish tinge, or, in 

 other words, becomes blended with yellow. Letter from Arago 

 toHumboldt; Cosmos, vol. iii. 



BEAUTY OF TWILIGHT. 



This phenomenon is caused by the refraction of solar light 

 enabling it to diffuse itself gradually over our hemisphere, ob- 

 scured by the shades of night, long before the sun appears, even 

 when that luminary is eighteen degrees below our horizon. It 

 is towards the poles that this reflected splendour of the great 

 luminary is longest visible, often changing the whole of the 

 night into a magic day, of which the inhabitants of southern 

 Europe can form no adequate conception. 



HOW PASCAL WEIGHED THE ATMOSPHERE. 



Pascal's treatise on the weight of the whole mass of air 

 forms the basis of the modern science of Pneumatics. In order 

 to prove that the mass of air presses by its weight on all the 

 bodies which it surrounds, and also that it is elastic and com- 

 pressible, he carried a balloon, half-filled with air, to the top 

 of the Puy de Dome, a mountain about 500 toises above Cler- 

 mont, in Auvergne. It gradually inflated itself as it ascended, 

 and when it reached the summit it was quite full, and swollen 

 as if fresh air had been blown into it ; or, what is the same 

 thing, it swelled in proportion as the weight of the column of 

 air which pressed upon it was diminished. When again brought 

 down it became more and more flaccid, and when it reached 

 the bottom it resumed its original condition. In the nine 

 chapters of which the treatise consists, Pascal shows that all 

 the phenomena and effects hitherto ascribed to the horror of a 

 vacuum arise from the weight of the mass of air ; and after ex- 

 plaining the variable pressure of the atmosphere in different 

 localities and in its different states, and the rise of water in 

 pumps, he calculates that the whole mass of air round our globe 

 weighs 8,983,889,440,000,000,000 French pounds. North- 

 British Review, No. 2. 



It seems probable, from many indications, that the greatest 

 height at which visible clouds ever exint does not exceed ten 

 miles ; at which height the density of the air is about an eighth 

 part of what it is at the level of the sea. Sir John Herschel. 



VARIATIONS OF CLIMATE. 



History informs us that many of the countries of Europe 

 which now possess very mild winters, at one time experienced 

 severe cold during this season of the year. The Tiber, at Rome, 

 was often frozen over, and snow at one time lay for forty days 



