1865.] 10]^ [Chase. 



Prague observations, that in winter the luuar-diurnal variation either 

 disappears, or is entirely concealed by irregular fluctuations, requir- 

 ing a long series for their diminution. The method of reduction 

 which he employed was, however, less perfect than that now used. 

 The second characteristic of the inequality consists in the earlier oc- 

 currence of the maxima and minima in winter than in summer. 

 The winter curve precedes the summer curve by about one and three 

 quarter hours." [Ibid. p. 12 — 13.] 



XI. The curves of semi-annual variation, at all points where con- 

 tinuous observations have been made, present a striking similarity, 

 the amount of deflection being nearly equal in all parts of the globe. 

 [See diagram, Pt. II, p. 12. Comp. St. Helena Obs., 2, cxix; To- 

 ronto Obs. 2, xvi, xvii.] 



Now it is evident that the mechanically-polarizing currents (Proc. 

 A. P. S., IX, 367 sqq), must be variously deflected at diff"erent pe- 

 riods of the day, in consequence of the gravitation disturbances 

 arising both from the varying distance of the sun, and from solar 

 heat. Although it may sometimes be desirable to consider the so- 

 called thermal currents apart from those which would be produced 

 independently of any change of temperature, such a distinction is 

 unnecessary in a preliminary qualitative investigation like the present, 

 because the periods both of maxima and of minima are precisely co- 

 incident in the two currents (except as they may be slightly modified 

 by the earth's radiant heat), and because even the thermal currents 

 are occasioned simply and solely hy the varying gravitation of fluids 

 of varying density. 



Regarding, therefore, the air and sether over any given magnetic 

 meridian, during the day hours the intertropical, and during the 

 night the extra-tropical, portions will be most drawn towards the 

 sun, and the following deflections will be thus produced in the por- 

 tions nearest the equator : 



6 to 12 A. M. 12 to 6 p. M. G to 12 p. m. 12 to 6 a. m. 

 Northern zones, . S. E. S. W. S. E. S. W. 



Southern " . N. E. N. W. N. E. N. W. 



The night disturbances, whether from variations of temperature, 

 or from simple fall towards the sun (the distance fallen varying as 

 the square of the time from midnight), will be very slight. The 

 earth's rotation, centrifugal force, and atmospheric inertia, tend to 

 throw each of the phases forward, and to increase the magnitude of 

 the westerly, while they diminish the easterly deflections. If these 



