76 
MR. C. CHAMBERS ON THE DIRECTION AND INTENSITY 
3. Annual Mean Values and Secular Change . — The annual mean values and yearly 
increments are as follows 
Table II. 
Year. 
Mean Dip. 
Annual Increase of Dip. 
1867-88 
19 2-3 
+1-9 
1868-69 
19 4-2 
1869-70 
19 61 
+ 1-9 
Mean Dip for three years. . . 
19 4-2 1 
The two yearly differences agree in making the annual secular change of dip a steady 
increase of T‘9 ; hence the mean dip for the three years, 19° 4 ,- 2, will properly correspond 
to the mean epoch, which is the 1st October, 1868. 
4. Annual Variation . — The average monthly values of dip for the period of three 
years, shown below, are affected by secular change ; this is eliminated, on the assump- 
tion that the change is uniform from month to month, by subtracting from the values 
for the months May to March respectively once, twice, thrice, &c. the monthly 
secular increase (0'T6). 
Table III. 
Month 
April. 
May. 
June. 
July. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
Jan. 
Feb. 
Mar. 
Mean Monthly Dip for 1 
period of three years... J 
19+ 
3-7 
41 
40 
40 
4-3 
4-2 
4-3 
4-3 
3-9 
4-0 
4-7 
5*0 
Correction for secular 1 
change, to April 15, 1868 J ’ 
0 
-0-2 
-0-3 
-0-5 
-0-6 
-0-8 
-10 
I 
-11 
-1-3 
-1-4 
-1-6 
-1-8 
Monthly values cleared of \ 
secular change J 
19+ 
3-7 
3-9 
3-7 
3-5 
37 
3-4 
3-3 
3-2 
2-6 
2-6 
31 
_T.I 
The means of different groups of the monthly values thus cleared of secular change, 
but affected by the annual variation of dip, are : — 
For April to September 
„ October to March 
„ May to August 
„ March, April, September, and October . 
,, November to February 
19 3-6 A 
19 3-0 
19 3-7 
19 3-4 
19 2‘9- 
Corrected for secular 
change to epoch 
April 15, 1868. 
which all agree in indicating an annual variation, in which the dip is a maximum when 
the sun is about its most northerly position in declination, a minimum when the sun 
is about its most southerly position, and having intermediate values when the sun is 
near the equator. The semiannual excess of dip when the sun has north declination 
above the mean value for the year is O'' 3. 
