1876.] 



Observations made at Stonyhurst College. 



319 



This Table gives the winter period an excess of 1' 1" over that of 

 summer, which agrees well with the mean of the values 1''31 and 0''80 

 found by Gren. Sabine and by Mr. Whipple from the Kew observations. 

 The value obtained by Dr. Stewart for the period 1863 to 1869 is 

 less than any of the above, and the result from the Stonyhurst observa- 

 tions taken during the same period is considerably below that of Dr. 

 Stewart; but still every series makes the winter number greater than 

 that for the summer. 



Table III. — Eesidual errors in the monthly values of the Dip. 



Summer 

 months. 



1870. 



1871. 



1872. 



1873. 



1874. 



1875. 



Mean. 



Semi- 

 annual 

 mean. 



April 



May 



June 



July 



September ... 



-5 24 

 +2 10 

 -0 12 

 + 1 46 

 -0 52 

 + 3 13 



-6 1.3 

 -1 52 

 -2 21 

 — 1 43 

 -1 26 

 -3 10 



+6 56 

 -2 45 

 -0 24 

 — 1 37 

 -1 46 

 +1 6 



-6 1 

 +3 12 

 +1 28 

 +0 7 



+5 33 

 +2 4 



+ 1 47 

 -2 49 

 +3 13 

 +2 32 

 +0 45 

 + 1 23 



-3 5 

 -1 10 

 -0 57 

 —2 8 

 +0 20 

 +3 23 



-1 6 

 -0 32 

 +0 8 

 —0 11 

 +0 26 

 + 1 20 



, +0 1-8 



) 



Winter 

 months. 



1870-71. 



1871-72. 



1872-73. 



1873-74. 



1874-75. 



1875-76. 



Mean. 



Semi- 

 annual 

 mean. 



October 



November ... 

 December ... 



February ... 

 March 



+6 27 

 +0 39 

 +0 50 

 +2 56 

 -1 31 

 -4 48 



-7 1 

 -4 44 

 -3 23 

 -1 54 

 -1 6 

 +1 17 



+6 2 

 -^0 10 

 + 1 10 



-0 33 

 +0 50 

 -0 10 



+6 55 

 +1 49 

 + 4 55 

 +1 15 

 -2 4 

 + 2 54 



+6 59 

 4-1 56 

 +0 49 

 + 1 42 

 -3 

 +0 20 



-3 43 

 , -2 17 

 -0 12 

 +4 

 +2 56 

 +3 11 



-1 24 



-0 28 

 +0 42 

 + 1 14 

 -0 39 

 +0 27 



1. -6 1-3 



Yearly means 



-0 4 



-2 18 



-0 17 



+1 51 



+0 40 



+0 2 





Erom these figures we conclude that the probable error of each monthly 

 value is + 1' 40", whilst that of the mean is + 0' 12". 



The Total Force. 



We are now in a position to test the semiannual variation of the 

 Total Eorce, or the Intensity of the earth's magnetism. Eeferring back 

 to Table I., we find for the summer periods the mean Horizontal Force 

 = 3-6321, and the mean Dip = 69° 30' 8" ; whilst for the ™ter periods 

 we get 3'6362 and 69° 29' 35". If, then, we apply to the winter epoch 

 the necessary corrections for secular variation, we obtain for the common 

 epoch of January 1st, 1873 — 



VOL. XXV. 2 A 



