78 
ME. C. CHAMBEES ON THE DIEECTION AND INTENSITY 
From January 1 to March 31, and from October 1 to December 31, 1868, 
0=0'-O'-l m+0'-3, ] 
And from April 1 to Sept. 30, 1868A 
O'T m-0'-3, j 
m being the excess of the date (in months) 
over July 15, 1868, — 
6 in every case representing the dip at the common epoch of the period, and 3' the 
observed dip. 
The weekly values of dip being thus corrected, the differences were taken between 
them and the mean of all the corrected values in each period, and to these differences 
the method of least squares was applied to find the probable errors for the several 
periods ; these are : — 
For April 29 to August 16, 1867, the probable error = + 0-67, 
,, August 23 to December 31, 1867 „ „ = + 0‘26, 
„ January 1 to December 31, 1868 „ „ = + 0 , 24. 
The remaining observations, of 1869 and 1870, being of about the same quality as 
those of 1868 will have about the same probable error. 
The unusual smallness of the probable error of a weekly determination since August 
17, 1867, is attributable mainly, I think, to the comparative infrequency and moderate 
amount of disturbance at a low latitude station, but partly also to the extreme care 
that has been given to secure the perfect preservation of the axles of the needles 
the needles themselves also appear to be of excellent character. 
7. Differences with different needles. — The mean dip observed with needle No. 2 
has, at different periods, exceeded that observed with needle No. 1, as shown below : — 
Table V. 
Period. 
Excess of Dip with Needle No. 2 over that 
with Needle No. 1. 
Morning observations. 
Afternoon observations. 
1867, June to August 
+5-0 
+5*0 
„ September to December 
+ 03 
-0 2 
1868, January to December 
+ 1-2 
+ 1-2 
It is noticeable that the great change between the two first periods is contem- 
poraneous with the introduction of the practice of remagnetizing the needle before 
commencing an observation. 
8. Monthly Mean Values of the Dip from April 1870 to March 1874. — 
Table VI. 
Year. 
April. 
1 
May. 
June. 
July. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
Jan. 
Feb. 
Mar. 
O 
19+ 
o 
19+ 
18+ 
o 
19+ 
19 + 
19+ 
o 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
1870-71 
10'-6 
12'-4 
8'0 
7'-0 
2'-5 
r-3 
7'-4 
6 ,- 4 
8'-8 
l'-9 
12-8 
12'-7 
1871-72 
14 1 
7-0 
0-9 
5-7 
9-3 
11-5 
9 7 
9-2 
11 -2 
11 -5 
11 -9 
11 7 
1872-73 
12-8 
14-3 
14-4 
15 0 
14-9 
14-9 
15 -3 
14 -7 
15-6 
14 -4 
14 -4 
16-7 
1873-74 
16-7 
16-4 
15 -6 
14 -6 
12-8 
12-9 
14 -2 
13-7 
13 -7 
14-2 
14 -9 
13 -8 
