544 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
tion of a lunation ; but there are several reasons against the hypothesis of a lunar 
cause in this case, besides the duration. The nearness of our satellite would 
render it extremely probable, if she were the cause of this variation, that the 
amount of the variation should depend upon the latitude of the place and the 
moon’s declination ; we shall see that it does not depend on the latter, and pro- 
bably little on the former. In the next place, we would expect some relation 
between the amount of the mean diurnal variation and the change of daily mean 
values. Such a relation exists between the solar diurnal variation and the change 
of daily mean value; but no such relation exists between the latter and the 
lunar diurnal variation, or it is but slightly marked; the amount of the change 
for this period is too large to be explained by the cause which produces the lunar 
_ diurnal variation. On the whole, I think we cannot attribute this period to 
the moon; and the range of the period is so considerable as to render it probable 
that the sun is the cause. The true period of rotation of the sun is about 25°63 
days, and the synodical rotation with reference to the earth is about 27°22 days; 
the latter number differs little from the first two series of well-marked groups in 
1844, and the former differs little from the mean as obtained from the two years. 
A careful investigation of a much larger series of observations leads me to 
believe that the period is variable within certain limits; and I think it may be 
possible, when this additional series of observations is examined, to show a pro- 
bable cause for this variation: meanwhile, without entering into any theory, I 
shall proceed to notice some results to be derived from the quantities under 
consideration. 
87. We have already considered the differences of the daily means, or the 
changes from day to day at different places; we may now examine the ranges 
of the best-marked of the 26-27 day period. The following are the approximate 
ranges (mean of rise from minimum to maximum, and of fall from maximum to 
minimum) of these movements where best marked at the four places in 1844 :— 
TABLE XXV.—RANGES OF 26-27 Day PERIOD: JANUARY TO APRIL 1844. 


PERIOD. Makerstoun, | Trevandrum. Singapore. Hobarton. 





8.39 5-93 5-07 5-50 
12°81 11-49 8-96 8-60 
12°93 9-48 8-68 8-88 
Jan. 9 to Feb. 5, . 
Feb. 5 ,, Mar. 6, . 
Mars O%,.. opiates 























34:13 26°99 22°71 22:98 


Sum, 



| ——— —— 


1:49 dou bi 0:99 1-00 





Ratio to Hobarton, . 


