VARIATION OF tHE COMPASS. g^^ 



for the last three or four years, the alteration has been so very 



imall, as to make it somewhat doubtful whether it may not be 



tonsidered stationary, but I would notfiom so short a period 



conclude that it real'y is so. 



From the observations of sixteen months, viz. from Sep- Remarks on 



tember, 1786, to December, 1737, both inclusive, the varia- '["^ changes of 



the varuition, 

 tion may be considered as generally stationary at or about 7 or s^c. 



8 o'clock in the morning, when it is least ; and about 1 or 2 



o'clock in the afternoon, when it is greatest ; and therefore it 



has been the practice in determining the true variation, put 



down in the tables, to take a mean of the two morning, and 



the two afternoon observations, made at those times, for the 



true variation. 



In March, 1787. The mean monthly diurnal alteration of 

 variation was found to be 15',0 ; in June 19'6 ; in July 19',6 j 

 in September 14 ',8; and in December 7 ',6. But on a mean 

 of 12 years observations, from the year 1793 to 1805, the 

 diurnal alteration of variation in March was only 8 ',5; ta 

 June 11 ',2; in July 10',6 ; in September 8 ',7 ; and in De- 

 cember 3 ',7 . 



Table IV. contains the differences for 1 2 years, viz. from Table of state 

 1793 to 1805, between the observations of the variation made of the variation 

 in the months of March, June, September, and December, or n^Janerly*"* 

 at the times of the vernal and autumnal equinoxes, and 

 summer and winter solstices; by a mean of these 12 years, 

 the variation appears to increase or go westward, from the 

 winter solstice to the vernal equinox 0',SOj diminishes or 

 goes eastward from the vernal equinox, to the summer sot* 

 slice 1 ',43; increases again from the summer solstice to the 

 autumnal equinox 2', J3 ; and continues nearly the same, only 

 decreasing 0'14, from the said equinox to the winter solstice. 



These differences at the time of tlie equinoxes and solstices 

 have been noticed by M. Gassini, in his observations made 



at 



iipping-ncedle in the same years. All the iron braces- were on the 

 north-west side of the needle, and the nearest about 13 feet from it. 



The allowances made to the observations of the variation, and also 

 of the dip, for the elfe'Ct of the iron-work of the room, were both as- 

 certained after the' above-mentioned alteration in the iron-wgrk took, 

 place; but they have, notwithstanding, been applied to the obser^Tif 

 jions made before, as Well as since that time. 



