90 



The Rev. H. Lloyd on the Results of Observations made at the 



19. The phenomena just described are, it is manifest, the resultants of two 

 distinct changes, — namely, the annual variation properly so called, and the secular 

 change. The amount of the latter is + 0'-5 x n,n being the number of months 

 elapsed. If this be subtracted from the numbers in the last row of Table X., 

 reducing to the epoch July 1 (the middle of the year), we obtain the numbers 

 of the following Table, which represent the course of the true annual variation. 

 The positive numbers correspond to easterly deviations, as before. 



Table XI. Periodical part of the Mean Annual Variation. 



The following is the equation of the curve of the annual variation, in which 

 .V = n X. 30°, n being the number of months, and parts of a month, reckoned 

 from January 1. We may probably neglect, as inconsiderable, all the terms 

 after the second. 



AD = 2'543 sin {x + 52° 57') + 0'-301 sin (2a: + 232° 33') 

 + O'-108 sin (3^- + 117° 46') + 0'-112 sin (4x + 26= 25') 

 + 0'-057 sin {5x + 295° 7') + 0'-005 sin 6x. 



The curve itself is represented in Plate III. fig. 6, the scale being 0.2 inch 

 to one minute of arc. For the sake of the comparison with the annual curve 

 of temperature, presently to be referred to, the signs are all changed, and the 

 positive ordinates correspond to westerly deviations. 



It appears from the inspection of this curve, that the course of the annual 

 variation (imUke that of the diurnal in this respect) is represented by a single 

 oscillation. The minimum occurs in the beginning of February, and the maxi- 

 mum in the beginning of August ; and the whole range of the change is 5-0 

 minutes. The curve crosses the axis of the absciss® in the middle of May and 

 in the beginning of November. 



To obtain the mean value of the declination corresponding to any month 

 in any year, the value of AD, obtained above, must be added to that of D, 

 given in Art. 14. The formula, therefore, is 



