172 THE BORDERLAND OF SCIENCE. 



or at least repulsive force exerted by the sun. They 

 have been seen to form like jets from a fountain, 

 rushing forth with incredible velocity till they have 

 attained their greatest height, and then falling back, 

 more or less rapidly, towards the sun's surface. Now 

 estimates have been formed respecting their velocity by 

 direct observation, the powers of the spectroscope 

 availing for this purpose. But we have in the height 

 to which the prominences attain, the most satisfactory 

 evidence of the velocity with which the gas comprising 

 them has been propelled from the sun's interior. 

 Assuming only that the prominences are formed of pro- 

 jected matter, we can as certainly determine the rate 

 of propulsion as we can determine at what rate a 

 cannon-ball must be propelled vertically upwards from 

 the earth's surface to reach a given height. At least, 

 we can determine the lowest velocity which would 

 suffice, supposing we see the full height of the promi- 

 nences, and nothing happens to check the upward 

 motion of their gaseous substance. But as surely as 

 we know that a cannon-ball must be fired at a much 

 higher velocity to attain a given height through the 

 air than would suffice if it could be fired in a vacuum, 

 so also we can infer that whatever velocity we deduce 

 for the prominence matter, regarding it as projected 

 through vacant space, must fall very far short of the 

 real velocity. May we not even go farther, and con- 

 sider rather the velocity with which the smoke leaves 

 the mouth of the cannon as compared with that neces- 

 sary to carry a projectile only to the height reached by 



