570 MR J. A. BROUN ON THE 
the beginning of each month are projected Plate XX XIX. From this we obtain 
the maximum 1818'2=8"79; the minimum 1813°7 = 696 2 
15. As the yearly mean range is so nearly constant from 1817°7 to 18185, 
the epoch of maximum is probably near the truth. The minimum may have 
occurred earlier.* 
16. Cassini's Observations.—The earliest systematic series with which we are 
acquainted showing a maximum or minimum is that made in the years 1784 to 
1788 by Cassin1, who employed CouLomp’s silk fibre suspension The astro- 
nomer of Paris, with the aid of three assistants, followed the movements of the - 
needle from morning till evening between 1783 and 1792;+ the maximum 
westerly position was obtained for each day from the observations between 
noon and 3 p.m., the mean of these positions was found for each eight days, 
corresponding to the 4th, 12th, 20th, and 27th or 28th of each month In a 
similar manner the minimum westerly positions were deduced from the morning 
or evening observations; the differences of four pairs of means thus calculated 
give the monthly mean ranges. The yearly means, corresponding to the Ist of 
each month, are projected Plate XX XIX. From this curve we derive the 
maximum 1787:°25=15':29; the minimum 1784:°8=9/:21 2 
As in the case of BEAuFoy’s observations the minimum noted is so near the 
beginning of the series that the exact epoch is by no means certain.§ 
* The ranges at Greenwich from 8" 40™ to 1* 20™, and at Makerstoun from 8" 40™ to 14 10™, 
about the minimum year 1844 were approximately as follows— 
Greenwich. Makerstoun. 
SAS 2 toocemaee TOO "Se Mibete em Ge 
WG Ard oi alee carers GO e ee ee eer OO 
SAD nena see BP ORI Le PPS 
We must conclude then, that if the minimum occurred at 1813°7, it had a greater value than in 
1844°5 at Greenwich ; or, if the value of the former minimum was nearly the same as the latter, that the 
former occurred probably near 1813°0. 
It may also be remarked that the mean of the daily ranges for Greenwich in 1847 was 1778, for 
which year the range from 8 40™ a.m, to 1" 20™ p.m. was approximately 8'°66, which is less than the 
maximum of 1818.- The mean of the daily ranges in 1818 was therefore between 18’ and 19’, From 
omitting the consideration of the hours to which Brauroy’s ranges refer, it has been supposed that the 
diurnal oscillations was very small in 1818. 
+ Observations Astronomiques et Physiques faites & YObservatoire en l’année 1791, p. 345 and 
note p. 350. Only the observations from 1783 to 1788 were published in Casstnr’s memoir “Sur la 
déclinaison et les variations de l’Aiguille aimantée lu & Académie Royale des Sc., Aofit 1871. While 
the results obtained by Cassrn1 for the mean position of the needle were vitiated by different causes, 
the deduced ranges are probably approximately true. 
{ Different authors have supposed erroneously that Cassinr observed only on these four days in 
each month. 
§ It is difficult in this instance to seek the epoch from the value of the range, as it is not quite 
certain whether CAssmnr’s ranges are those of the means for each week at the hour of least and greatest 
declination, or simply the means of the daily ranges. If the former, then the minimum was probably 
less than that noted, and occurred earlier. It should also be pointed out that we are not acquainted 
with any marked period, such as that of Cassry1, in which the minimum occurs only 24 years before 
the maximum. 


