ON MAGNETIC DISTUKBANCES AND EARTH-CURRENTS. 471 



2 H and S V are expressed in millimetre- milligrammes for 1 milli- 

 metre, or in centimetre-grammes for 1 centimetre, i.e. thej are the values 

 of a centimetre (a unit of length) of the ordinate, according to the C.G.S. 

 system of units. 



From this table it appears that the same absolute disturbance will 

 produce changes in the ordinates of the St. Petersburg horizontal force 

 curve, which are more than four times the corresponding changes in the 

 ordinates in the Kew curve, and which are nearly six times the corre- 

 sponding changes in the ordinates in the Bombay curve. 



The scale-values in this table are the same as in the originals, except 

 in the case of Zi-ka-Wei and Toronto. The values for oH in the original 

 curves are -00062 millimetre-milligrammes for Zi-ka-Wei and "OOOSS for 

 Toronto. 



A comparison of the curves with the scale-values shows that at the 

 beginning of the storm on August 11 the change in the horizontal force 

 at Vienna was very nearly equal to the change at St. Petersburg, and 

 about 1^ times the change at Zi-ka-Wei ; also the changes at Melbourne, 

 at Zi-ka-Wei, at Toronto, and at Bombay are very nearly equal to one 

 another, and amount to about '0047 millimetre-milligrammes. 



Thus the amount of the disturbance does not seem to increase with 

 the latitude. Again, the changes occurring between 11.30 and 11.40 a.m. 

 amount to about — 



•016 mm.-mgr. at St. Petersburg; 



•013 „ at Vienna and at Zi-ka-Wei ; 



•010 „ at Kew and at Toronto, but in opposite 



directions ; 

 •0084 „ at Bombay. 



Thus the absolute value of the disturbance at Bombay is not much 

 less than in higher latitudes, the flatness of the curve being due to the 

 fact that the needle is not sufficiently sensitive. On comparing the curves 

 with the table of scale-values, it seems clear that for magnetic storms the 

 most satisfactory curves for comparison and for measurement are those 

 which do not differ much in size from the Vienna curves, i.e. the most 

 satisfactory scale-value is about '0004 or 'OOOS millimetre-milligrammes 

 for 1 millimetre. When the scale- values are very much larger, as in the 

 Kew and Bombay instruments, the needles are not sensitive enough to give 

 the character of the disturbances, and when the scale-value is less than 

 about "0004 millimetre-milligrammes, the curves may have a very wide 

 range, and are very liable to go off" the edge of the paper. 



Great differences of practice exist at different observatories ; thus, at 

 St. Petersburg and at Toronto, a curve with ordinates increasing upwards 

 corresponds to decreasing horizontal force. This was also the case at 

 Vienna in 1879 ; but, from a comparison of the curves for August 1880, 

 it appears that a change has been made by which a curve with ordinates 

 increasing upwards corresponds to inci'easing horizontal force. At all the 

 other observatories a curve with ordinates increasing upwards corresponds 

 to increasing horizontal force, and it would facilitate comparison if this 

 rule were universally observed. In the Lisbon and Coimbi'a and in the 

 Melbourne curves only the times of the beginning and end and one or 

 two other points in the curve are determined ; on this account some diffi- 

 culty has been found in comparing them with the others. At other obser- 

 vatories, at every hour or at every two hours, the time is marked by an 



