DIRECTION AND VELOCITY OF THE WIND. 215 



of the angles of direction we find tliat the effect of the 

 additional force which acts upon the wind from 7 a.m. to 

 9 P.M. was to impel the air through a distance of 16*3 

 miles in a direction from S. 69° 36' W. to N. 69° 36' E. 

 Now this direction is almost exactly that of a perpendicular 

 to the magnetic meridian. Referring to the Greenwich 

 Magnetical Observations, we find that the magnetic decli- 

 nation in the years 1859- 1865 ^^^ ^^ follows : — 



o / 

 1859 21 43-5 W. 



i860 21 i4'3 „ 



1861 .... 21 5-4 „ 



1862 20 52*0 „ 



1863 20 46*0 „ 



1864 20 38-0 „ 



1865 20 35-0 „ 



The mean for the seven years is 20° 56'* 3 W., a perpen- 

 dicular to which is S. 69° 3'7 W. to N. 69° 3'7 E., thus 

 differing only 32'*3 from the above determination This 

 close relation to the direction of the magnetic meridian, 

 taken in connexion with the points of agreement between 

 the phases of the two classes of phenomena already ad- 

 verted to, seems to establish beyond doubt the fact that 

 the force which produces the diurnal inequality in the 

 direction and velocity of the wind, and the earth's mag- 

 netic force, are in some way intimately connected with and 

 dependent upon each other. 



It has, I believe, been generally supposed that the mean 

 position of the magnetic needle for the 24 hours of each 

 day indicates the direction of the true magnetic meridian, 

 or is that in which the needle is subject to the least 

 amount of disturbing force, and that its diurnal oscilla- 

 tions are due to two forces acting alternately in opposite 

 directions — or to one force, the acting centre of which 

 changes its position so that during part of the 24 hours it 

 is on one side of the magnetic meridian, and during the 



