1869.] 



Survey of the West of France. 



487 



Dip. Decl. H. F. 



Paris 65-875 17*841 4*1133 



Laval 65-802 19 073 44245 



Brest 66-460 21-005 4*0442 



Vannes 65-585 20 225 4-1328 



Angers 65-140 19*093 4-2106 



Poitiers 64*468 18*306 4*2955 



Bordeaux 63*383 18-209 4-4110 



Abbadia 62'463 18*235 4'5456 



Bayonne 62-503 18*391 4*5520 



Pau 61-970 17*825 4*5823 



Toulouse 62-018 17*122 4*5883 



Perigueux 63*398 17*682 4'4268 



Bourges 64-543 17*003 4*2845 



Amiens 66-672 18*316 4*0143 



Secular Variation. . .. — 3'*68 Z + 0'0050 \ / — 9'*1 \ 

 Acceleration 0-043 \ 0*00002/ 2 \ 0-19/, 



The secular variation has been obtained by comparing the observations 

 of this survey with those of Dr. Laniont, taken about ten years previously. 



Maps of the isodynamic, isoclinal, and isogonic lines of the epoch, Sep- 

 tember 1st, 1868, are drawn from the following data, Paris being chosen 

 as the central station for reasons given in the paper : — 



For the isoclinals the direction is 



N. 73° 25' 10" E. to S. 73° 25' 10" W., 

 the distance between the lines being 44*25 geographical miles for a change 

 of 30' of dip. 



The direction for the isogonics is 



N. 20° 31' 16" E. to S. 20° 31' 16" W., 

 and the distance only slightly greater than for the isoclinals, i. e. 44*35 

 geographical miles for 30' of angle. 



The isodynamics lie in the direction 



N. 70° 34' 13" E. to S. 70° 34' 13" W., 

 the distance in this case being 115 geographical miles for a change of 0*1 

 in the intensity. 



For the lines of equal horizontal force the direction is 

 N. 74° 19' 30" E. to S. 74° 19' 30" W., 

 and 72 geographical miles the distance separating lines where the horizontal 

 intensity differs by 0*1. 



An attempt has been made to apply a correction for the magnetic dis- 

 turbances at the times of observation by means of the magnetograrns ob- 

 tained at Stonyhurst Observatory during the Survey ; but these corrections 

 have not been taken into account in forming the equations of condition 

 from which the final results have been obtained. 



The probable error of any single observation of the dip, declination, total 

 force, and horizontal component are found to be respectively 

 3'-13 ; 0'*95 ; 0*0144 ; and 0*0067. 



VOL. XVII. 2 o 



