10 



AMPLITUDE OF THE SOLAR-DIURNAL VARIATION 



maximum reading of western declination. In the last two columns the difference 

 of the scale readings, or the amplitude of eastern and western elongation, is made 

 out in scale divisions, and also in minutes of arc. 



The inequality of this amplitude next requires our attention. 



The inequality constituting the ten or eleven year period is plainly exhibited in 

 the last two columns of the above table, the progression in the numbers being quite 

 regular. The year 1843 is clearly indicated as the year of the minimum range of 

 the diurnal fluctuation, but whether the period is one nearer to ten or to eleven 

 years cannot be decided from the Girard College observations, since they do not 

 embrace a year of maximum amplitude. The epoch of the minimum, however, 

 can be determined with more precision. For this purpose only, the values in the 

 last column are represented by the formula, 



A = 9'.08 1'.14 (t -- 1840.5) + 0'.201 (t 1840.5) 2 , 

 deduced by the method of least squares, and the quantities come out as follows : 



Probable error of any single amplitude, +0'.ll. 



That portion of the ten or eleven year period which results from the preceding 

 discussion of the differential observations of the magnetic declination, free from the 

 effect of the disturbances as far as the latter can be eliminated, is shown graphically 

 in Fig. 1. 



Middle of each year. 



