ATMOSPHERIC TIDE AT ST. HELENA. 
49 
It should here be remarked, that as the observations were made at hours of solar 
time, they could not of course strictly correspond in the majority of cases to the 
lunar hours under which the value of b — b have been arranged. The effects of this 
imperfect synchronism have a tendency to compensate each other, and are probably 
nearly compensated, at all the lunar hours except at 0 and 12, 6 and 18. These, 
however, are the most important hours for the determination of the maximum effect, 
and, supposing there to be no establishment, it is obvious that whether the observa- 
tion precedes, or whether it follows the precise lunar hour to which it should corre- 
spond, the error produced will be the same in kind, viz. the maximum at 0 h and 12 h 
will be lowered, and the minimum at 6 h and 18 h will be raised ; and in both cases 
the error will tend to diminish the apparent influence of the moon’s position on the 
barometric pressure. It may therefore be presumed that the true horary variation 
corresponding to the lunar hours is in every case greater than the numbers which 
appear in the preceding table ; and that the horary variation at 0 h and 12 h which 
should express the whole difference of pressure corresponding to the moon’s positions 
on the meridian and at six hours’ distance from it, is especially less than the true 
amount, being diminished by the causes above mentioned, both at the hours when 
the moon is on the meridian and when she is six hours distant. To obviate this in- 
convenience and to give the results in future as much precision as they are capable 
of, eight additional observations, at equal lunar intervals, will hereafter be made in 
each day, corresponding precisely to the lunar hours of 0, 3, 6, 9 21 ; and these 
observations will be made on Sundays as well as on other days. 
We may conclude therefore as the result of the two years of observation from 
October 1843 to September 1845 inclusive, that the barometer at the Observatory at 
St. Helena is higher when the moon is on the meridian, either above or below the 
pole, than when she is six hours distant from the meridian, by an average quantity 
which exceeds - 00365 in., and may be taken in round numbers as •004 in. 
For the purpose of examining whether any perceptible difference in the influence 
of the moon on the barometric pressure could be detected at the periods of the 
apogee and perigee, the following method w r as adopted. The epoch of the moon’s 
perigee or apogee being taken from the Nautical Almanac, the nearest of the lunar 
hours 0, 6, 12 , or 18 to that epoch is taken as the middle term of comparison ; if it 
be 0 or 12 hours, the values of b — b at the four antecedent and four subsequent 
lunar meridian hours (0 and 12) are taken in addition to the middle term, to give a 
mean value corresponding to the times when the inoon is on the meridian above or 
below the pole. The mean of the eight intermediate values of b — b at 6 and 18 hours 
taken in like manner, furnishes a mean value corresponding to the times when the 
moon is six hours distant from the meridian. The difference between these mean 
values of b — b gives for the epoch in question, the excess of the barometrical pressure 
when the moon is on the meridian above the pressure when she is six hours distant 
from it. Theoretically, this excess should be greater at periods of perigee than at 
those of apogee. 
MDCCCXLVII. 
H 
