68 Variation of the Magnetic Needle. 
so that it was necessary to subtract this quantity from all the 
observations to obtain the true variation. The mean varia- 
tion for each hour of the day, and for each month of the 
year, as deduced from these observations, and corrected for 
the error 3’ 25”, are given in the following tables. 
Mean Varia- Mean Variation 
Time. tion for the Hour. /from April, 1810, 
to May, 
6h.A.M.| 6° 19 Q1’ 
1810, April, ee ait oi" 7 6 19 7 
ay, 6 23 36 8 6 19 09 
June, G 2s 42 9 6 20 28 
J uly, 6° 28 -51 10 Se 2i 1D 
August, 6 29 “44 11 6°22 46 
September, (6 25 21 12 6 24 07 
October, O21. 42 IP. MG. ae 40 
November, (6 19 11 2 6 27 09 
December, 6 12 35 3 27 00 
1811, January, S20 “So 4 oC me oF 
February, (6 21 19 5 6 24 26 
March, So. 30° 9 6 a Sa te 
April, S23 oo 7 o. 21, op 
May, S 2i* 38 8 21-9 
9 6 20 54 
10 6 QW ae 
The whole number of observations was 5125, and the 
