162 FROM NEBULA TO NEBULA 



turn, described by Sir John Herschel in Articles 542 and 

 550, respectively (Outlines of Astronomy) : 



An extremely singular relation subsists between the mean 

 angular velocities or mean motions of the three first satellites of 

 Jupiter. If the mean angular velocity of the first be added to 

 twice that of the third, the sum will equal three times that of the 

 second. From this relation it follows that if from the mean 

 longitude of the first added to twice that of the third, be sub- 

 ducted three times that of the second, the remainder will always 

 be the same or constant, and observation informs us that this con- 

 stant is 1 80, or two right angles ; so that the situation of any two 

 of them being given, that of the third may be found. * * * One 

 curious consequence is, that these three satellites cannot be all 

 eclipsed at once; for, in consequence of the last mentioned rela- 

 tion, when the second and third lie in the same direction from the 

 centre, the first must lie on the opposite. * * * 



A remarkable relation subsists between the periodical times 

 of the two interior satellites of Saturn and those of the two next 

 in order of distance; viz., that the period of the third (Tethys) is 

 double that of the first (Mimas), and that of the fourth (Dione) 

 double that of the second (Enceladus). The coincidence is ex- 

 act in either case to about 1-800 part of the larger period. 



That the atmosphere should be affected by tides in 

 much the same manner as the hydrosphere goes without 

 saying, but on account of its invisibility, its tidal be- 

 havior cannot be so exactly investigated and defined. 

 One point of this relationship is revealed by the ob- 

 served fact that when the ocean level is low, the barom- 

 eter is always higher than normal. It is atmospheric 

 tides that constitute the chief cause of trade winds, but 

 I shall not weary the reader by discussing this collateral 

 subject in the present work. 



TERRESTRIAL MAGNETISM 



By substituting, in theory, tides of depression for 

 lifting tides, we not only gain in frictional effect, but 

 we likewise transpose the locus of it to the floors of the 

 oceans instead of to their coasts. On the other hand, I 

 have given reasons to justify the deduction that the 

 metals, being the heaviest substances, are mainly cen- 

 trally located within the earth, and ranged along its 

 axis. The semblance of the earth, therefore, to a helix 



