ON THE MAGNETIC DECLINATION. 13 



inferred from the ten-year series of the Prague observations, that in winter the 

 lunar-diurnal variation either disappears, or is entirely concealed by irregular fluc- 

 tuations, requiring 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 occurrence of the maxima 

 and minima in winter than in summer. The winter curve precedes the summer 

 curve by about one and three-quarter hours. Both these features are well ex- 

 pressed in the above diagram. At Toronto, the same shifting in the maxima and 

 minima epochs was notice'd, but the other inequality in the amount of deflection 

 is not exhibited. It seems probable that the Philadelphia results are more typical 

 in form than those either of Prague or Toronto. It is also apparent that the 

 smaller deflection at the upper culmination in the annual mean, when compared 

 with the deflection at the lower culmination, is entirely produced by the feeble 

 lunar action in winter. The maximum west deflection in summer occurs actually 

 near the upper culmination. At the same season the maximum east deflection is 

 still retained (as in the annual curve) about six hours after the lower culmination. 

 In the winter season this last mentioned maximum east deflection is actually the 

 smaller of the two. We have 



Maximum summer range 35".4, Secondary, 31".8 



" winter " 25 .2, " 15 .6 



Difference 10 .2, 16 .2 



At Prague the maximum summer range was 44". 



Next I proceed to examine whether the phases of the moon, the declination, or 

 parallax, have any sensible effect upon the magnetic declination. Mr. Kreil found, 

 from a ten years' series of observations at Prague, that there was no specific change 

 in the position of the magnet depending upon the moon's phases and parallax, but 

 that the declination was 6". 8 greater when the moon was at the greatest northern 

 declination than when at the greatest southern declination. On the contrary, Mr. 

 Broun, from the Makerstoun observations, a much shorter series than the one at 

 Prague, inferred that there was a maximum of declination two days after the full 

 moon. He also found a maximum corresponding to the greatest northern declina- 

 tion of the moon, but does not appear to have investigated the effect of distance. 

 The residuals which we have been treating enable us at once to examine these 

 several points. 



Beginning with the lunar phases, the daily means for the day of full and new 

 moon, and for two succeeding days, were compared with the monthly mean declina- 

 tion. In case any of the hours were disturbed, the monthly normal for the hour 

 was substituted for the disturbed observation before the mean was taken. If one- 

 half or more of the hourly readings were disturbed, the daily mean was altogether 

 omitted. Accidental omissions of hourly observations were supplied by the hourly 

 normal. The half-monthly normals were then compared with the half-monthly 

 means. In the table of differences thus formed, equal weight is given to the 

 bi-hourly and hourly observations. The daily mean having been subtracted from the 

 monthly mean, the positive sign indicates a western deflection, and the negative sign 

 3 



