62 SCIENCE IN SHORT CHAPTERS. 



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 me- 

 chanical 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 disc of the radio- 

 meter is surrounded with a considerable quantity of atmos- 

 pheric matter is probably due to inequality of atmospheric 

 pressure. The absorbing face of the disc 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. Crookes's estimate of the mechanical energy 

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

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

 they are offered, i. <?., merely as provisional and approxi- 

 mate 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 estimated 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 ac- 

 tion, 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 increasing with the subdivison of masses, 

 and becomes an important fraction in the case of the smaller 

 bodies of the solar system. A zone of meteorites traveling 



