50 
ON THE LUNAR ATMOSPHERIC TIDE AT ST. HELENA. 
If the lunar hour nearest to the epoch of apogee or perigee be 6 or 18 instead of 
0 or 12, that hour is taken as the middle term of comparison, and the mean values 
of b—b consist in such case of the mean of nine values at 6 and 18 hours, and of 
eight at 0 and 12. When a Sunday intervenes the same number of values of b—b, 
viz. eight of the one and nine of the other, are taken to give the mean value in the 
comparison, which consequently in such case extends to a somewhat greater distance 
on either side of the middle term than when no such interruption occurs. 
The mean excess of barometrical pressure when the moon is on the meridian, 
resulting from this comparison, is as follows : 
From 13 epochs of perigee between October 1843 and September 1844 ‘00407 in. 
From 13 epochs of perigee between October 1844 and September 1845 ‘00394 in. 
From 13 epochs of apogee between October 1843 and September 1844 ‘00341 in. 
From 14 epochs of apogee between October 1844 and September 1845 ‘00347 in. 
The number of observations from which each of the first three results is obtained 
is 221 ; in the fourth case it is 238. 
We have here in both years a consistent indication of the greater influence of the 
moon on the barometrical pressure at periods of perigee than at those of apogee. 
The effect of the solar action on the atmospheric pressure is far more difficult to 
be shown by any mode of grouping the observations than is that of the moon, not 
only because it is much smaller, but because it is masked in the diurnal barometric 
oscillation by the much greater and variable influence of the solar heat in producing 
atmospheric variations. That it is eliminated, in common with the other variations 
which depend on the sun’s horary angle, by the process described in this paper, may 
be inferred from the fact that no significant difference appears in comparing the 
excess in the values of b—b at the lunar hours of 0 and 12, at the periods of syzygies 
and quadratures, during the twelve months from October 1843 to September 1844 
inclusive. The mean values in this comparison have been taken from the observations 
at 0 h and 12, 6 and 18 for 36 hours preceding, and 36 hours following, the epoch of 
each syzygy and quadrature. The mean excess at 0 and 12 at the periods of syzygy 
is ‘00337, and at those of quadrature *00345. 
The quantities treated of in this communication will justly appear to many persons 
as extremely small ; but the consistency of the partial results, when the observations 
are broken into periods of six months, places beyond doubt the power of observation, 
in appropriate circumstances and continued for a sufficient time, to determine both 
the existence and the approximate amount of a diurnal systematic affection of the 
barometer, not exceeding four thousandths of an inch. It also affords an instructive 
example of the beneficial influence of mean numerical values in the advancement of 
the physical sciences, and of the power which we possess through their means of 
progressively separating from apparently irregular phenomena, that which is constant 
and reducible to laws. 
