ATMOSPHERIC TIDE AT ST. HELENA. 
47 
Variation of the Barometer at the several Lunar Hours, from Oetober 1842 to Sep- 
tember 1843 inclusive, in decimals of an inch, -f- signifying 1 an excess of baro- 
metric pressure, and — a defect. 
1st. From the superior to the inferior passage. 
Moon on 
the upper 
meridian . 
+ lb. 
+ 2&. 
+ 3f>. 
+ 4b. 
+ 5b. 
+6s. 
+ 7 h . 
+ 8&. 
+9». 
+ 10&. 
+ iu. 
+•0014 
+•0011 
+■0002 
•0000 
-•0008 
-•0010 
-•0012 
-•0018 
-•0011 
-•0005 
+•0002 
+■0012 
+ 0012 
2nd. From the inferior to the superior passage. 
— 12* 1 . 
-lib. 
-10&. 
_ 9 h. 
-8b. 
— 7 h - 
-6&. 
-5b. 
_4h. 
-3b. 
-2b. 
— lb. 
Moon on 
the upper 
meridian. 
+■0016 
+•0012 
+•0009 
+•0002 
-•0006 
-•0010 
-■0011 
-•0010 
-•0004 
+•0004 
+•0003 
+•0016 
+•0014 
We have here an average excess of barometric pressure of ‘0014 in. at the hour 
when the moon is on the meridian above or below the pole, and an average defect of 
•00115 at the period when she is six hours distant from the meridian ; making together 
an average difference in the height of the barometer of ’00255 in. between the hours 
when the moon is on the meridian and when she is six hours distant from it. 
The arrangement of the observations for the investigation proving rather a heavy 
charge on the establishment at St. Helena, the examination was now taken up at 
Woolwich, and carried through a subsequent period of two years, i. e. from October 
1843 to September 1845 inclusive, for which the following method was adopted : — 
If we call b the height of the barometer at 32° at any hour of observation, and b 
the mean height of the barometer at the same hour during the month to which the 
day belongs, then b—b is a quantity which remains over after the approximate 
diurnal variation has been eliminated ; it is + when the barometer is higher and — 
when it is lower than the normal state, and expresses by how much the barometer is 
higher or lower at that observation than the normal state at the same hour. Monthly 
tables are then formed in which are inserted the values of b—b for each hour of mean 
solar time. The mean solar hours which are respectively nearest to the several lunar 
hours are then computed for every day, and the values of b—b are re-arranged in 
lunar monthly tables. The means at the several lunar hours in each month are then 
taken ; and these means are finally arranged in periods of six months, yielding mean 
values of the barometrical variation at the several lunar hours for each half year. 
These are shown in the following tables : — 
