MAGNETIC DECLINATION AT TREVANDEUM. 751 



42. The regular double maximum and minimum shown in all the mean 

 results obtained for the lunar diurnal variation of the magnetic elements has 

 given the idea that the variations are due to the action of an attractive force ; 

 any such idea will now require some modification such as that suggested (37) 

 in order to satisfy the facts of this paper.* 



POSTSCRIPT.! 



43. In the preceding paper, no allusion has been made to the results when 

 related to the moon's phase ; these phases coinciding nearly in each month with 

 one of the four positions of the moon in declination, for which the curves had 

 been projected, it did not seem necessary to give the curves for the phase also. 

 It has been pointed out (foot note to 32) that the day and night hours overlap 

 each other when the means for several successive days are taken, and therefore 

 the separation of the part of the curve derived from day hours, from that 

 derived from the night hours, is not perfectly definite. 



This, however, is the case in these combinations, not only from the mode of 

 combining several successive days, but also because, for example, in the month of 

 January, the moon does not pass the meridian at the same hour for the day she 

 is farthest north in different years, the difference amounting to upwards of three 

 hours in the course of the eleven years. It seemed desirable, then, to avoid this 

 latter complication, in order to show more distinctly the difference betwixt the 

 movements during the day and during the night hours. This was best done in the 

 combinations for the moon's phase, since the moon is on the meridian at noon 

 nearly on the day of new moon, and at midnight nearly on the day of full 

 moon ; the only indefiniteness remaining is that common to both combinations, 

 depending on the overlapping in the means of successive days already noticed. 



After communicating the results of the preceding paper to the Royal Society, 

 I projected the curves for each month corresponding to the new moon, first 

 quarter, full moon, and third quarter. That is to say, the means derived from 



* The mean diurnal variations of the atmospheric pressure due to the sun and moon, show similar 

 regular double maxima and minima. The consideration of a mass of observations made by me in India 

 at different heights, induced me several years ago (Phil. Mag., August 1858) to suggest an electro-mag- 

 netical attraction as a cause of these variations (and I found an analogical phenomenon in the action of 

 the sun on the gases of comets). The atmosphere having acquired a certain polarity, would by this attrac- 

 tion assume ellipsoidal forms with the longer axis directed towards the attracting luminaries (or making 

 nearly constant angles with these directions), thus producing waves or currents as a given meridian 

 turns round under axes of different lengths. I have in consequence of this idea sought to find some 

 connection between the laws of terrestrial magnetism and those of atmospheric pressure, but hitherto with- 

 out any conclusive result. The difference of the lunar action on the magnetic needle during the day and 

 during the night, suggested the examination of the lunar diurnal variation of atmospheric pressure for a 

 like difference ; but the result of this investigation was negative, no difference being perceptible ; the lunar 

 diurnal variation in January and June was shown equally well by the observations during the night 

 and by those during the day. 



"j" Added by permission of the Council, June 27th 1872. 



VOL. XXVI. PART IV. 9 K 



