Certain Questions connected with Astronomical Physics. 265 



current carbon arc in air and carbon-aluminium arc in air, 

 we can obtain oscillations of high frequency. 



Much further investigation is, however, necessary to enable 

 us to state precisely the action of the surrounding medium, 

 hydrogen, or better coal-gas, in producing this steep charac- 

 teristic volt-ampere curve for the carbon-metal arc in these 

 gases. 



XXI. On Certain Questions connected ivith Astronomical 

 Physics. — Part II. By S. Tolver Preston, Ph.D.* 



IT appears to be a physical question of interest, demanding 

 a settlement, as to whether a planet with direct rotation 

 (as illustrated by our earth), or with retrograde rotation (as 

 exemplified by Neptune, with its satellites), should be the 

 normal result to be expected from the familiar Nebular 

 Hypothesis of Kant and Laplace. 



In Sir George Darwin's Presidential Address f before the 

 British Association in 1905, we find the passage : — 



" The concentration of an ill-balanced or broken ring on 

 an interior point would necessarily generate a planet with 

 direct rotation — that is to say, rotating in the same direction 

 as the earth. But several writers, and notably Faye, en- 

 deavour to show — erroneously as I think — that a retrograde 

 rotation should be normal . . . But I do not claim to have 

 removed the difficulty, only to have shifted it , for the satel- 

 lites of Neptune, and presumably the planet itself, have 

 retrograde rotations ; and lastly, the astonishing discovery 

 has just been made by William Pickering of a ninth retro- 

 grade satellite of Saturn, while the rotations of the eight 

 other satellites, of the ring and of the planet itself, are direct." 

 (< Nature/ August 31, 1905, p. MO.) <£? 



Preceding the above Address no longer than the date 

 April 27, 1905, indicates, a letter under title " A little-known 

 Property of the Gyroscope/'' by Prof. William H. Pickering 

 (of Harvard Observatory), appeared in ' Nature/ where he 

 seems to regard it as an already accepted view that a retro- 

 grade rotation of the planets of the solar system should be 

 normal. Since any evidence (direct or indirect), based on 

 observation, that might contribute to confirm the one or the 



* Communicated by the Author. The first part (under same tifle) 

 appeared in the Philosophical Magazine for December 1906. 



t Part I. of this Presidential Address is to be found in '-Nature/ 

 August 17, 1905. 



Phil. Mag. S. 6. Vol. 14. No. 80. Aug. 1907. T 



V 



