1881.] bol [Chase, 



115. The Milky Way and Mikller's Hypothesis. 



Sir John Herschel* objects to Miidler's assignment of "the local centre 

 in space, round which the sun and stars revolve ' ' to the group of the 

 Pleiades, "lying as it does no less than 26° out of the plane of the galactic 

 circle, out of which it is almost inconceivable that any general circulation 

 can take place." No such objection can be urged against the radius-vector 

 of the Centaurean paraboloid, for the Milky Way divides at a. CerUauri,* 

 and it traverses Cassiopeia, "its brightest part passing about two degrees 

 to the north of the star d of that constellation."* 



116. Daily Variations of the Magnetic Needle. 



Sabine's discussions of the magnetic observations at various stations have 

 shown : 



1. That the diurnal variation of declination which is due to the Moon's 

 action consists of four equal or nearly equal portions, in which the mag- 

 net is attracted alternately to the east and to the west of its mean position. 



2. That there is a striking correspondence between the lunar-daily vari- 

 tions of horizontal force and of declination. 



3. That in the normal variations of vertical force, the lunar day is also 

 divided into four alternating periods of nearly equal duration, in two of 

 which the force is increased and in the other two it is diminished by the 

 lunar influence. 



4. That the lunar-daily variations of inclination and of total force also 

 constitute double progressions, having two maxima and two minima, with 

 alternate periods of increase and decrease, each period being of about six 

 hours' duration. 



5. That the solar-daily variations normally constitute only single pro- 

 gressions, of about twelve hours each, from maximum to minimum and 

 from minimum to maximum. 



6. That there are, however, "nocturnal episodes" of "retrogressive 

 motion" at some stations, with tendencies to a triple instead of double 

 division of the solar day. 



I have shown, by experiment : 



7. That any elongated body, when exposed to the action of parallel 

 rectilinear undulations, tends to place itself in the line of those undula- 

 tions. 



8. That this tendency may be increased by giving the elongated body a 

 slight specific energy of direction. For example : if the gimbals of a bin- 

 nacle compass are so held as to allow motion only in one direction, and 

 the box is jnade to swing on its free axis like a pendulum, the needle will 

 tend towards the line of oscillation. 



The single progression of the solar disturbances (5) and the double pro- 

 gression of the lunar disturbances (1 — 4) indicate a difference in the char- 

 acter of the chief disturbing influences of the two bodies. It is difiicult to 



* Op. cit. Sect., 861, 789, 787. 



