75 



to a deflection of the north end of the magnet to the east of its mean 

 place, and the sign to the west. 



Kew A#=+0"-64-2"-54 sin(0 + 6 '2) 9"'74sin(2a + 59 '8). 

 HobartonA < r=-0"-H-l"'14sm(a+344 7) + 6"-8sm(2+43-2). 



In computing the lunar-diurnal variation by means of these for- 

 mulae, the coefficient of the term which includes the sine of twice the 

 hour-angle is of principal importance : the subsequent terms are 

 comparatively of little significance, and are therefore omitted on the 

 present occasion. When all the terms are employed, the original 

 observed values are reproduced. 



Table I. exhibits, at Kew, in column 2 the lunar-diurnal variation 

 as actually observed on the mean of the three years, and in column 3, 

 the same computed by the formula. Column 4 is the lunar- diurnal 

 variation at Hobarton on the mean of the five years as observed, and 

 column 5 the same computed by the formula. 



TABLE I. Lunar-diurnal Variation at Kew and Hobarton. 



