MARKINGS OF MERCURY ANTONIADI 103 



An analysis of past observations has shown me that all the most 

 important spots drawn by De Ball, Denning, Schiaparelli, Jarry- 

 Desloges, Fournier, and Dan j on are confirmed by my own observa- 

 tions ; and that the principal markings of my map have been partly 

 confirmed with the 33-inch by Messrs. Ritchey, Lyot, Burson, Baldet, 

 Grenat, Roger, Mile. Roumens, and by M. Swings, of the University 

 of Liege, in Belgium, observing with me Mercury in the large 

 instrument. 



We thus have the converging evidence of many hundreds of mutual, 

 independent, confirmations of the existence of definite spots on Mer- 

 cury by several well-trained observers, and this always, without excep- 

 tion, in the positions necessitated by a period of uniform rotation 

 equal to the period of revolution. Hence the SB"* rotation of the planet 

 is now demonstrated to be established on an immovable basis. 



The idea of a rapid rotation, of some 24^, rests merely on analogy 

 with the kindred rotation of Mars and of the Earth ; and, apart from 

 the decisive conflicting evidence of observation, it has the further 

 disadvantage of ignoring the important action of the bodily tides. 

 Now, I have shown that Mercury is with reference to the Sun in a po- 

 sition comparable with that of lapetus with regard to Saturn;^ and 

 we know, since the days of Cassini, that lapetus, apart from his libra- 

 tions, presents always the same face to Saturn. The mean distance 

 of lapetus to his primary is 62 mean radii of Saturn ; and the mean 

 distance of Mercury to the Sun is 83 solar radii. But the density of 

 the Sun is double that of Saturn ; and, applying the sixth power law, 

 which governs the frictional force slowing down rotation, we find 

 that Mercury, as above stated, is with the reference to the Sun in a 

 comparable position with that of lapetus to Saturn. The duration 

 of such tidal actions comes here into play, and somebody may ask if 

 the Sun is as old as Saturn. A cosmogonist, skilled in the arts of 

 grasping the exact manner in which the various globes of the universe 

 were begotten, can alone clear up that mysterious question. 



The application of the sixth power law to the satellites has further 

 enabled me to demonstrate that all those bodies known up to 1893 

 show, apart from their librations, always the same face to their pri- 

 maries.® The problem had to be set in a very particular way, to which 

 I was led by observation ; and that is the reason why the law govern- 

 ing the rotation of the satellites had eluded the penetration of such 

 profound mathematicians as Henri Poincare and Sir George Darwin. 



The same law will enable us to understand the very reason of the 

 fundamental difference existing between the rotation of the planets 

 and that of the principal satellites; expressing distances in radii of 



* Letter to Mrs. Maunder, Jour. British Astron. Assoc, vol. 39, p. 86, 1928. 

 •Bull. Soc. Astron. Prance, vol. 43, pp. 385-398, 1929. 



