ON CURVES OF DECLINATION MAGNETOGRAPHSi, ETC. 205 



From the Stony liurst report Ave find that ' the chief disturbance of the 

 month began about noon on the 9th, and lasted till 4 a.m. on the follow- 

 ing day.' 



On comparing the Lisbon and Coimbra curves for the aa hole period of 

 this disttirbance, they are found to be absolutely coincident throughout. 



On comparing the Kevv and Stonyhurst curves, they are also found to 

 be absolutely coincident, both in I'ange of disturbances and in time ; indeed, 

 this is one of the most remarkable instances that I have seen. 



At Vienna the disturbances are nearly all of the same character, and 

 take place at the same time, but the i'ange is not quite so great. 



On comparing the St. Petersburg curves, it is found that there are 

 disturbances of the same character, and taking place — i.e. having their 

 maxima and minima— at the same time as those at Kew and Vienna and 

 the other stations ; but superposed upon these are other disturbances, one 

 to the eastward fi-om 2 to 3.20 p.m., and to the vrestward from 8.20 to 

 3.40 p.m. ; another violent one to the eastv^ard from 4.20 to 4.50 p.m., 

 followed by a quicker retui-n to the westward until 5 p.m. ; another, not 

 quite so violent, eastward from 6 to 6.30, and westward from 6.30 to 

 7 p.m. ; then, after a period of comparative rest, at 10.20 there is another 

 disturbance westward for about ten minutes, followed by a i-eturn of 

 the needle to the eastward until 11 p.m., superposed on those dis- 

 turbances which are the same as the disturbances which are seen in the 

 Kew curves. 



The effect of these extra disturbances, which are so marked at St, 

 Petersburg, is only just seen in the Vienna curves, the result being that 

 the heights of some of the maxima are diminished or increased, or the 

 slopes of parts of the curves are slightly altered, in consequence of the 

 action of opposing or reinforcing disturbances. 



These differences in the disturbances at St. Petersburg and at the 

 other stations coincide in time with corresponding changes in the value 

 of the horizontal force, as measured by the Kew curves. Thus from 2 to 

 3.20 p.m. the horizontal force is diminishing, then from 3.20 to 3.40 p.m. 

 the horizontal force is increasing ; from about 4.15 to 4.45 p.m. the hori- 

 zontal force is diminishing, but again increases more rapidly until 5 p.m.; 

 then from 6 to 6.20 p.m. it diminishes, and afterwards increases more 

 slowly until a little after 7 p.m.; after a period of rest there is a large in- 

 crease from 10.15 to 10.35, followed by a diminution of the horizontal 

 force until 11 p.m. 



It thus appears from these comparisons — and the statements are fully 

 borne out by the other principal disturbances which have been examined 

 — that : 



A diminution in the horizontal force is accompanied by greater easterly 

 deflections of the declination needle at St. Petersburg than at Kew. 

 2. Increase of the horizontal force is accompanied by greater westerly 

 deflections at St. Petersburg than at Kew, or is sometimes accompanied 

 by a westerly deflection at St. Petersburg and an easterly deflection at 

 Kew. 



On March 11, a disturbance, first eastward for a quarter of an hour 

 until 9 p.m., then westward for an hour, causes a well-mai'ked and regular 

 depression in the declination curve. 



This takes place at the same instant at Kew, Stonyhurst, and Vienna, 

 but is not present at St. Petersburg; but at the time of the greatest 

 eastward deflection, at 9.4 p.m., there is a slight westward deflection at 



