342 DE. C. CHKEE: ANALYSIS OF KESULTS FROM THE KEW .HAGNETOOEAPHS 
decidedly Ijelow the average. Again, we have specially large clianges of H at Kew 
and Potsdam in 1892-3, 1895-6, 1896-7, and 1899-1900. Parc St. Maur agrees 
with Potsdam and Kew in having a specially small change of H in 1893-4, and 
specially large changes in 1892-3 and 1896-7 : bnt it differs in showing an average 
change in 1891-2 and a less than average change in 1895-6. 
In inclination and vertical force the mean data at the three stations are widely 
different; Parc St. Maur shows a nearly stationary value of Y, as against a distinctly 
risino- value at Potsdam, and a still more distinctlv fallinij value at Kew. The 
inclination is diminishing at all three places, hut the rate of fall appears considerably 
less at Potsdam than at Parc St. Maur, and considerably less at Parc St. Maur than 
at Kcav. 
Speaking generally, the secular changes in I, Y. and T appear to have been much 
more irren'ular at Potsdam and Pai’c St. j\Iaur than at Kew. Thus, at Parc St. Maur 
V I'ose 29y in 1891-2 and fell 40y in 1893-4 ; while at Potsdam Y increa.sed by 40y 
in 1891-2, 43y in 1892-3, and 74y in 1899-1900, and fell 16y in each of the three- 
years 1893-4, 1894-5, and 1898-9. There are corresponding fluctuations, of course, 
in T. At Kew the accuracy of the estimate of secular change in Y and T is really 
dependent in the main on the accuracy of the determination of I. An error of P 
in I—and such an error might arise from very slight deterioration of either circle or 
needles—means an error of over 30y in both Y and T. Taking this into account, I do 
not think much weight can he assigned to the values of the secular change for 
individual years. The comparative smoothness of the Y and T data in Table III. is 
probably not more a tribute to the uniformity of Nature than to the care and skill of 
Mr. Bakkr as an observer. 
The theory Isas been advanced that secular changes are particularly large in years 
<.)f sun-spot maximum. Tlie data in Talde 111. can hardly be said to support this. 
The changes of D were certainly larger in 1892-6 when sun-spots were numerous, 
than in 1898-1900 when they were few; hut the largest change in D occurred in 
1 890-91 near sun-spot minimum, and in 1893-4 about sun-spot maximum the secular 
change was about average in D and particularly small in H and I. 
§ 8. To assist in the compreliension of the secular cliange I have drawn a curve, 
fig. 1, which shows the change from year to year in the direction of an imaginary 
magnet, freely suspended from its C.G. so as to point along the line of total force. 
The curve may he supposed drawn by a style attached to the dipping end of the 
magnet, the paper being supposed to lie on a ])lane tangential to a sphere, concentric 
with the magnet, at the point answering to the mean position of the dipping end 
during the period 1891 to 1900. ^Yith increasing I, tlie dipping end would move 
towards an observer wlio stood witli his hack to the south, looking at the ])a})er. 
The west, i.e., the direction of + I), would then he on the observer’s left. The 
crosses mark the positions of the end of the magnet at the middle of the years 
specified. Tlie magnet or stvle is sup|)osed to he so long that the difference between 
