38 
THE REV. STEPHEN J. PERRY ON THE MAGNETIC 
lations, since the time occupied in the survey was not long enough to cause any consider- 
able change to take place during the interval. The epoch for all the observations may 
therefore be taken as September 1st, 1868. Table YI. contains a comparison between 
the above results, and those published by Dr. Lamont in his ‘ Erdmagnetismus. ’ 
Table YI. 
Station. 
Dip, Jaip 1, 185S. 
Sept, 1, 1868. 
Biff, of Epoch. 
Diff. of Dip. 
Yearly rate 
of decrease. 
Dip on Jan. 1, 
1869. 
Paris 
66 26-5 
65 53-75 
10-8 
— 32-75 
303 
o / 
65 52 5 
Angers 
65 55-9 
65 9-7 
-46-2 
4-28 
65 8-4 
Poitiers 
65 8-3 
64 29-4 
-38-9 
3-60 
64 28-1 
Bordeaux ... 
64 5-S 
63 24-2 
-41-6 
3-85 
63 23 0 
Bayonne 
Toulouse ... 
63 6-S 
62 31-4 
• 
-35-4 
3-28 
62 30 2 
62 46-1 
62 2-2 
» 
-43-9 
4-06 
62 1-1 
Mean 
3-68 
Supposing the same rate of decrease to hold for the remaining stations, where Lamont 
did not observe this magnetic element, we have for the 1st of Jan. 1869 the following 
results : — 
results : — 
Laval 65° 48T Pau ei 58-2 
Brest 6626'4 Perigueux 63 23’9 
Vannes 65 47 T Bourges 64 32-6 
Abbadia 62 27*8 Amiens 66 40 '3 
The dip was observed on March 3rd, 1866, by M. d’Abbadie at his magnetic obser- 
vatory, and found to be 62° 39'T5, which gives 4'*22 for the annual decrease. 
Referring, now, to former determinations of the secular decrease of the magnetic dip at 
Paris, we find, from a memoir by M. G. Aime, “ Sur le magnetisme terrestre”*, that the 
mean annual decrease between the years 
1671 and 1754 was L7 
1754 and 1780 „ 1*0 
1780 and 1806 „ 6*0. 
From 1780 to 1830 the yearly diminution in the decrease amounted to 0' - 051, as we 
learn from a Table in General Sabine’s article in the Report of the British Association 
for 1838. 
Comparing, now, the results of the Dip Observations taken in 1858 and 1868 with the 
mean of those obtained by Arago, Humboldt, and Mathieu from 1825 to 1830, we find 
the annual decrease of the dip at Paris to be 2'-82 for 1843, whilst that for 1863-4 is 
3';68, which shows that there is at present a gradual acceleration in the decrease of this 
element amounting to about O'-043 per annum. Dr. Lamont gives 2 f, 7 as the annual 
diminution for 1858, which is somewhat smaller than the amount found above. 
* Published in the Annales tie Chimie et de Physique, 3 e serie, t. xvii. 
