236 THE NEW KNOWLEDGE. 



" weight" which the sunlight can balance, about half the 

 thousandth of a millimetre, would travel under the pressure 

 of light more that 865,000 miles an hour. In comets' tails 

 we probably have particles whose diameter is less than one- 

 eighteenth of this. Such particles would travel the same 

 distance in less than four minutes. 



It is not surprising, then, that " the tail of the great comet 

 of 1680 was found by Newton to have been no less than 

 20,000,000 leagues in length and to have occupied only two 

 days in its emission from the comet's body, a decisive 

 proof this of its being darted forth by some active force, 

 the origin of which, to judge, by the direction of the tail, 

 must be sought in the sun itself. " The whole matter is ac- 

 counted for by the mechanical pressure of light, a force in 

 the universe hitherto unsuspected. 



THE SOLAR PROMINENCES AND THE CORONA. 



In a solar eclipse, at the precise moment when the moon 

 blots out the sun's disc there becomes visible around the 

 edge of the sun a number of magnificent scarlet steamers, 

 or clouds, some of them 60,000 miles in height and held 

 suspended over the sun. These are the solar "prominences" 



In addition to these fiery streams there exists also a beauti- 

 ful halo or "glory" of a greenish or pearly lustre which 

 contrasts finely with the scarlet hue of the prominences. 

 This "halo" has been called the "corona" 



Both the prominences and corona consist of matter in a 

 highly rarefied condition and the so-far unanswered question 

 of the astronomers has been, " How is this matter held up?" 

 The complete answer seems to be, "It is held up by the 

 pressure of the sun's own light." The sun must project 

 vapours into space. These vapours will condense into drops 

 where they meet the cold of outer space. These drops, if 



