or THE EARTH’S MAGNETIC FORCE AT BOMBAY. 
77 
5. Diurnal Inequality at Observation-hours . — The difference between the morning 
and afternoon observations is almost inappreciable, as will be seen by the following 
statement, which embraces every observation in 1867 and 1868, finished before 15 hours 
or commenced after 15 hours, for every month in which observations of both kinds are 
recorded; the general result is a slight excess of dip in the afternoon: — 
Table IV. 
Mean Time of Observation. 
Mean 
Dip. 
Excess of 
Period. 
Needle 
No. 
Number 
of Obser- 
vations. 
First half. 
Last half. 
Afternoon 
over 
Morning. 
Commence- 
ment. 
Conclusion. 
Commence- 
ment. 
Conclusion. 
\ 1S67, June to December 
1 
17 
h m 
10 22 
h m 
10 56 
k m 
11 4 
h m 
11 47 
19 10 
} +0-3 
1 
13 
15 20 
15 50 
16 5 
16 38 
19 1-3 
2 
15 
10 23 
10 58 
11 19 
11 50 
19 3 3 
j 0-0 
2 
15 
15 26 
15 55 
16 11 
16 41 
19 33 
! 1868, January to December ... 
1 
26 
10 25 
10 49 
10 59 
11 25 
19 3-2 
} +0*1 
„ „ „ 
1 
26 
15 2-1 
15 53 
16 0 
16 25 
19 3-3 
„ „ 9f 
2 
27 
10 18 
10 44 
10 54 
11 21 
19 4-4 
} +0-1 
| „ 
2 
25 
15 24 
15 49 
15 59 
16 24 
19 4-5 
From this it may be inferred that the annual variation of the diurnal inequality of 
dip for the mean observation hour (or rather the mean variation for the morning and 
afternoon hours) is also probably small, and may scarcely affect sensibly the annual 
variation of dip found above ; this latter variation must, however, be accepted subject 
to correction, if necessary, when the annual variation of the diurnal inequality of dip 
has been well determined. 
6. Probable Error . — It will suffice to give an idea of the quality of the observations of 
dip if an account be here given of the results of determinations, made early in 1869, of 
the probable error of a single weekly determination (being the mean of a pair of obser- 
vations) in three distinct periods in 1867 and 1868. The semiannual inequality found, 
by a similar process to that described above, from this more limited body of observa- 
tions was the same as that found above from three years’ observations, viz. an excess of 
0 ,- 3 in the dip from April to September above the mean of the year, and an equal 
defect during the opposite half-year. The annual secular increase of dip was found to 
be l ,- 3. These values were embodied in formulae for correcting the observed weekly 
values of dip to a common epoch as follows : — 
From April 29 to August 16, 1867, 
0=&- O'T m, 
m being the excess (in months) of the date over June 15, 1867 ; 
From August 23 to September 27, 1867, 
6=& — O'T to — O'* 3 ; 
From October 1 to December 31, 1867, 
6=0- O'T to+0'*3, 
m 2 
m being the excess of the date (in months) 
1 1 over October 15,1867; 
