THE PHILOSOPHY OF THE RADIOMETER. 113 



These different modes of regarding the action are perfectly 

 consistent with each other, and analogous to the two different 

 modes of regarding gravitation, when we describe the sun as 

 attracting the earth, or, otherwise, the earth as gravitating to 

 the sun. Strictly speaking, neither of these descriptions is 

 correct, as the gravitation is mutual, and the total quantity 

 exerted betsveen the sun and the earth is equal to the sum of 

 their energies, but it is sometimes convenient to regard the 

 action from a solar standpoint, and at others from a terrestrial. 

 So with the radiometer and the strictly mutual repulsions 

 between it and the predominating radiant. 



It appears to me that this unsophisticated conception of 

 radiant mechanical repulsive force, and its necessary mechanical 

 reaction on the radiant body, meets all the facts at present 

 revealed by the experiments of Mr. Crookes and others. 



The attraction which occurs when the disk of the radiometer 

 is surrounded with a considerable quantity of atmospheric 

 matter is probably due to inequality of atmospheric pressure. 

 The absorbing face of the disk becomes heated above the 

 temperature of the opposite face, the film of air in contact with 

 the warmer face rises, leaving a relatively vacuous space in 

 front. This produces a rush of air from back to front which 

 carries the radiometer vane with it. When the exhaustion of 

 the radiometer is carried so far that the residual air is only just 

 sufficiently dense to neutralize the direct repulsion of radiation, 

 the neutral point is reached. When exhaustion is carried 

 beyond this, repulsion predominates. 



Taking Mr. Crookcs's estimate of the mechanical energy of 

 solar radiation at 32 grains per square foot, 2 cwts. per t cre, 

 57 tons per square mile, etc., and accepting these as they are 

 offered i.e. merely as provisional and approximate estimates 

 we are led to a cosmical inference of the highest importance, 

 one that must materially modify our interpretations of some of 

 the grandest phenomena of the universe. Although the esti- 

 mated sunlight pressure upon the earth, the three thousand 

 millions of tons, is too small a fraction of the earth's total 

 weight to effect an easily measurable increase of the length of 

 our year, the case is quite otherwise with the asteroids and the 

 zones of meteoric matter revolving around the sun. 



The mechanical repulsion of radiation is a superficial action, 

 and must, therefore, vary with the amount of surface exposed, 

 while that of gravitation varies with the mass. Thus the ratio 

 of radiant repulsion to the attraction of gravitation goes on 



