G. Sack: lieber die tägliche, jährliche und elfjährige Periode etc. 
5 
Nur die Zahlen der 5. Spalte sind für die Jahre 1883—1887 den „Results of Greenwich Observations“ 
Tabelle IX Spalte 2 entnommen, alle übrigen Angaben sind mir von Herrn Christie mit folgendem Begleit 
schreiben mitgetheilt worden: 
„ further details are given concerning methods of working up the results, applying 
them to reduction of the observations for 1887. 
The mean of the twelve monthly Horizontal Force ordinates by the registering instrument 
in the year 1887 for the times corresponding to the observations of absolute horizontal force is 
517, the mean of the corresponding twelve observed values of absolute horizontal force being 
1.8175. Taking the difference between the mean ordinate 517 and the several mean monthly 
values given in Table XI of the printed volume we obtain differences for each month which 
multiplied by 1.8175 give corrections to the mean yearly observed absolute horizontal force, by 
application of which mean monthly values are found as follows. 
Mean monthly 
Horizont. Force 
Table XI 
Excess 
above 517 
Excess X 1.8175 
= correction 
to 1.S175 
Mean monthly 
value of 
Horizont. Force 
467 
-50 
- 9 
1.8166 
471 
-46 
- S 
1.8167 
483 
-34 
- 6 
1.8169 
521 
+ 4 
+ 1 
1.8176 
5S0 
+63 
+11 
1.8186 
553 
+36 
+ 7 
1.8182 
597 
+80 
+15 
1.8190 
547 
+30 
+ 5 
1.S1S0 
607 
+90 
+16 
1.8191 
557 
+40 
+ 7 
1.8182 
533 
+16 
+ 3 
1.8178 
432 
—85 
-15 
1.S160 
Mean 1.8177 
Or we may take the twelve monthly horizontal force ordinates by the registering instrument 
for the times corresponding to observations of absolute horizontal force and take the difference 
between each and the corresponding mean monthly values (Table XI) so obtaining a correction 
which multiplied by 1.8175 gives for each month a correction to the observed value of absolute 
horizontal force, by application of which mean monthly values are found. 
HorizontalForce ordinate 
by registering instrument 
at time of observation 
of absolute horizont. force 
Monthly 
value 
Table XI 
Correc 
tion 
Corr. 
X 1.8175 
Observed 
absolute 
value 
Mean 
monthly 
value 
391 
467 
+ 76 
+14 
1.8145 
1.8159 
460 
471 
+ 11 
+ 2 
1.8125 
1.8127 
489 
483 
- 6 
- 1 
1.S148 
1.8147 
508 
521 
+ 13 
+ 2 
1.8175 
1.8177 
593 
580 
- 13 
_ 2 
1.8182 
1.8180 
663 
553 
-110 
-20 
1.8218 
1.8198 
611 
597 
- 14 
— 3 
1.8184 
1.8181 
635 
547 
- 88 
-16 
1.8204 
1.8188 
682 
607 
— 75 
-14 
1.8205 
1.8191 
463 
557 
+ 94 
+17 
1.8176 
1.8193 
400 
533 
+133 
+24 
1.8179 
1.8203 
313 
432 
+ 119 
+22 
1.8164 
1.8186 
Mean 1.S177 
The two methods should give the same results. The first depends on the steadiness of the 
bifilar magnet through the year, in the second any accidental error in the determination of any