142 MR BROTJN ON THE RELATION OF THE VARIATIONS OF THE 



the vertical component about the solstices and minima about the equinoxes; 1844 

 and 1845 also indicate a maximum at the winter solstice, but the maximum at 

 the summer solstice is imperfectly shewn, if shewn at all. 



14. It is quite evident that a moderate error in the temperature correction 

 may be sufficient to destroy all appearance of an annual period, especially when 

 the range of the temperature may be 30' Fahrenheit, and the range of the com- 

 ponent for the annual period may be small. It is, I am inclined to think, due to the 

 existence of a lighted stove in the Observatory in the winters of 1842, and in the 

 beginning of 1843 (by means of which the annual range of temperature was much 

 diminished), that these years give a somewhat distinct indication of the annual 

 period. For many reasons, however, it is to the observations made in the years 

 1844 and 1845 that I look for a consistent exhibition of the annual period ; and 

 to the result for these years I shall return when the insufficiency alluded to has 

 been remedied. 



15. The vertical component has diminished considerably since 1841, the 

 yearly rate of diminution becoming less in each year. Something of this apparent 

 change may be due to a loss of magnetism in the balance needle ; but it is believed 

 that this is only partially, if at all, the case. There is a curious change in the 

 rate of diminution of the vertical component in the year 1845 : in October, No- 

 vember, and December, it was constant or very nearly so ; it was remarked of the 

 horizontal component for 1845, that it had increased much less than in the pre- 

 vious year. Does this point to an approaching turning-point for the diminution 

 of the magnetic dip ? 



16. Similar summations, to those indicated for the horizontal component 

 (No. I., 25); were made for the vertical component, at the varying hour angles of 

 the moon ; the larger disturbances were also eliminated similarly, the test num- 

 ber for disturbances being taken more than twice as great in 1845 as in 1844. 

 From a mean of 12 lunations in 1844 (see Curves No. 3, Plate VI.), the maximum of 

 vertical component occurred when the moon had passed the inferior meridian 

 about three hours, the value of the component then diminished considerably till 

 19'' (counting the moon on the meridian 0'', and so up to 24'', when it wants one 

 hour, or, more exactly, II of an hour of being on the meridian again ; each of the 

 so-called hours having only this value, lO*" corresponds to about 5'' 45"" before the 

 moon's meridian passage) ; it diminishes slightly from 19'' till 22'', when there is a 

 minimum ; it then increases slightly till 24'' or O"" when there is a maximum ; after 

 this it diminislies moderately again till 7*", when the principal minimum occurs ; 

 it then increases rapidly to the maximum at 16". When the disturbances are 

 not eliminated the maximum occurs at IS*", the component then diminishes rapidly, 

 with some h-regularities, till 2'', which is .the time of the principal minimum ; a 

 secondary maximum then occurs at 4'', and a minimum, differing very little from 

 the other in value, occurs at 0" or 7". A more comjilete elimination of distur- 

 bances, it is conceived, M'ould render the maximum about O*" still more evident. 



