SUEVEY OE THE WEST OF FRANCE. 
47 
At Angers an iron pipe was lying N. and S. at a distance of about 11 yards from the 
place of observation. Neglecting, therefore, this station, we find the yearly decrease 
of the Declination in the West of France to be 9' 4"- 9. 
M. G. Aime, in the “ Memoire ” already quoted, gives the mean annual variation of the 
Declination at Paris for past years. 
Epoch. 
1663 to 1767 
1767 to 1785 
1785 to 1805 
1805 to 1817 
1817 to 1825 
Declination. 
0° o' to 19 16 
19 16 to 22 0 
22 0 to 22 5 
22 5 to 22 19 
22 19 to 22 22 
Yearly variation. 
- 4 - 11-0 
+ 9-0 
+ 0-2 
+ 1-1 
+ 0-3 
Continuing this summary a step further we have 
1825 to 1858, 22° 22' to 19° 36', yearly diminution 5'-0, 
1858-0 to 1868-7, 19° 36' to 17° 53', yearly diminution 9'*6, 
whence it appears that the declination is rapidly on the decrease, with a mean yearly 
acceleration in the decrease of O'- 22. 
The A-alue of the yearly diminution, as given by Dr. Lamont, is 7'- 6 for 1858, which 
shows a steadiness in the variation of this magnetic element. 
A glance at the maps which accompany this report will show at once the changes 
that ten years have produced in the position of the lines of equal declination, dip, and 
intensity. The distance between the lines remains in all cases almost constant for the 
same element, and the amount is moreover identical for the isoclinal and isogonic lines, 
the values of r being respectively 0 - 678 and 0-677. The angle moved through by the 
lines is more considerable in the cases of the isoclinal lines than for the others, and the 
direction of this motion is away from the astronomical meridian in the case of the iso- 
gonics and isoclinals, and scarcely perceptibly towards the astronomical meridian for 
the lines of equal horizontal force. 
In the maps of Lamont’s 4 Erdmagnetismus’ the lines are curved, and the epochs for the 
dip, horizontal force, and declination are severally August 1848, June 1848, and March 
1854; Avhereas I have taken January 1st, 1858, as the common epoch, and drawn the 
lines straight, for the sake of comparison with the lines for 1868, Avhich are laid doAvn 
Avithout any modification from the calculated values of u and r. The dotted lines, Avhich 
belong to the survey of 1858, can therefore only be looked on as first approximations to 
the results derived from Lamont’s observations. The broken lines are those obtained by 
the above calculations. 
The numbers marked at each station are the means of the observed values, and they 
serve to shoAV the degree of approximative correctness attained by the adopted method 
