142 MR BROUN 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, | 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. L., 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, #4 of an hour of being on the meridian again; each of the 
so-called hours having only this value, 19* 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 0" when there is a maximum ; after 
this it diminishes 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 15", the component then diminishes rapidly, 
with some irregularities, 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 6° or 7°. A more complete elimination of distur- 
bances, it is conceived, would render the maximum about 0° still more evident. 

