DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 579 
between 1773 and 1777. The last column of numbers has, however, definite 
limits, and they show very markedly the maximum in 1776, from which VAN 
_SWINDEN concluded the possible existence of a period of eight years. 
31. The observations of Corrf made at Montmorency have been employed 
by Dr Wotr and Professor Loomis. They give the following quantities for 
the diurnal range of the magnetic needle :— 
Lett : : 11’-2 LHC : : : 85 
1778 , : : 10°0 1780 5°5 
These show very distinctly that the maximum was not later than 1777, and 
that it occurred probably earlier. They thus confirm, as far as possible, VAN 
SWINDEN’S epoch. 
32. VAN SwWINDEN’s numbers show, also, a minimum in 1772, which agrees 
with the epoch found Art. 22. The epoch of the next following minimum is 
not certain, as the series ends with 1780.. The Manheim observations of the 
magnetic needle show a minimum at 1782°5, and the principal minimum at 
17845; Dr Wotr, by his spot numbers, finds 1784°8; Professor Loomis’s 
numbers of aurore are very irregular from 1779 to 1786, showing a minimum 
at 1780 and 1784'5. The forty-two year period gives 1782°2; and I think 
the determination of the preceding maximum gives some weight to this 
result. 
There are no means of testing the earlier epochs of Dr Wo.r; but no long 
period given by him will be satisfied by them. If I have already shown good 
grounds for substituting a maximum in 1776 for Dr WoLr’s minimum, a similar 
change in some of the epochs of the preceding century and half may be quite 
possible. 
33. Diurnal Ranges of the Magnetic Needle in the years of Maximum and 
Minimum, and their Ratios.—There is considerable difficulty at present in 
arriving at any satisfactory comparison of the ranges during different periods, 
derived, as they are, from observations made at different places and at different 
hours. We may, however, determine approximately the ratios of the greatest 
to the least oscillations in each case. They are as follows :— 


Place. Observer. Years. Ranges. Ratios. Observations. 
Paris, CassINI ; sa e a : =e 1:66 e* During the day. 
1829°7 13°74 
u Ee tee MOTE I0 Vie ae TS 4 
Gottingen, Gauss ; nee sje OOS 1:74 fT 8" a.m. to 1 p.m. 
18335% ° ~750? 
* As the minimum probably occurred earlier than 1784°8, the ratio here found is too small. 
+ The minimum at Gottingen is estimated from the curve, Plate XX XIX. 
VOL. XXVII. PART IV (er 
