100 FROM NEBULA TO NEBULA 



allow as well for the exceptions to the general rule : it 

 must be able to account, differentially, for the elongated 

 orbits of comets as satisfactorily as for the rotundity of 

 those of the planets; for the high inclinations of the 

 asteroidal orbits as for tfhe narrowness of the zodiac ; for 

 the rare cases of retrograde motion as for the vastly more 

 numerous direct. 



Doubtless the reader is familiar with the phenome- 

 non of the miniature whirlpool that forms on the surface 

 of a basin of water while escaping through a central 

 outlet in the bottom. Next the circumference the rota- 

 tion, as you know, is relatively slow, but it increases 

 rapidly toward the center, where often a hollow cone is 

 formed round which the water revolves with high speed. 

 Do you know the reason for this? It is because the 

 equilibrium of the water, as a whole, is constantly being 

 unsettled by the falling away of its support, and the act 

 of gyration follows as an automatic effort at a general 

 balancing. If the basin were very deep and the process 

 sufficiently prolonged, you might perhaps find it worth 

 while to investigate the law of gyration by sprinkling 

 some sawdust on the surface and timing the turns at dif- 

 ferent distances from the axis of rotation. I can tell you 

 the answer, namely, the cubes of the distances are as the 

 squares of the periodic times. 



This phenomenon has several curious counterparts 

 in nature in the ocean, in the atmosphere, and in the 

 skies. Thus Sir John Murray (TJie Ocean, p. 198) says: 

 "The Michael Sars Expedition of 1910 measured tidal 

 currents in the open ocean down to a depth of 400 

 fathoms. It was found that the currents at 274 fathoms 

 ran in the opposite direction to that of the upper layers 

 which again approached that of the currents at much 

 greater depths. At certain moments the currents appear 

 to be arranged in the form of a spiral staircase, the whole 

 system turning in clockwise direction from top to bot- 

 tom. ' ' Again, Professor W. I. Milham in his standard 

 work (Meteorology, p. 162), describes a similar peculi- 

 arity regarding the action of the atmospheric tides at 



