204 KEPORT— 1880. 



the same time (about 7. "20 p.m.) at all tbc stations. Theu the needle 

 gradually returns to the westward, and the disturbance dies away. 



This deviation of the St. Petersburg curves from the others occurs at 

 6.40 p.m., at which time there is a sudden increase in the horizontal 

 force. 



Another considerable disturbance, consisting of a general eastward 

 movement of the north end of the needle, began about 6 p.m. on the 7th, 

 followed by a westward movement, which ceased about 10 p.m. 



In this disturbance, as in others, the Lisbon and Coimbra curves are 

 like exact reproductions of one another, so also are the Kew and Stony- 

 hurst curves. Placing the Lisbon negative behind the Coimbra positive, 

 the dark lines of the Lisbon photograph are seen through the bright 

 lines of the Coimbra curves ; and in the same way, placing the Kew 

 negative behind the Stony hurst positive, the dark lines of the Kew curves 

 are seen to coincide with the bright lines of the Stonyhur.st cui-ves, just 

 as if one were an exact print taken from the other. 



Comparing the Kew and the \'icnna curves this disturbance is found 

 to be of precisely' flic same cliaracter at both stations, but its range at 

 Vienna is less than at Kew. In this case the periods of the disturbance 

 occur at the same absolute time at all the stations. 



At St. Petersburg Ihe disturbance at the beginning is also similar in 

 character to that at Kew, but previously at 2 p.m. (Greenwich time), 

 there had been an easterly disturbance at St. Petersburg, which was not 

 perceived at Kew; and just before 8 p.m., towards the end of the great 

 disturbance, the westerly range of the needle is very much greater at 

 St. Petersburg than at Kew, but the needle reaches its extreme positions 

 either west or east exactly at the same absolute time at the two, and, 

 indeed, at all the stations. 



Unfortunately at Coimbra four curves are drawn on the same slip, 

 and the zero line for one curve frequently runs into and coincides witli 

 the curve for another dnj, so that it is difficult or impossible to make out 

 the character of the disturbances. The distance between the curve and 

 the zero line appears to be the same as in the Kew curves. 



At Stonyhnrst three curves are photographed on the same slip, but 

 the difficulty of dealing with the Coimbra curves is avoided by placing 

 the zero or time line a long way from its own curve, but the curves for 

 different days are placed so close to one another that occasionally they 

 are apt to run into and confuse or cross one another. 



At Kew, at Lisbon, and at St. Petersburg, two curves are drawn on 

 the same slip, and sufficiently far apart not to interfere with one another, 

 the distance at St. Petersburg being greater than at Kew, because, as a 

 rule, the disturbances are of larger amount than at Kew. 



At Vienna each curve is photographed on a separate slip, and the 

 hours ai-e numbered astronomically from to 23, the slip being changed 

 at or just before 21 hr., or 9 a.m. local time, i.e. about 8 a.m. Greenwich 

 time. 



The Vienna plan of photographing each curve on a separate sheet is 

 the most convenient of all for the comparison of disturbances at different 

 places, and there is an additional advantage in this plan because when 

 there are two or more curves on a slip, disturbances occurring at the same 

 hour on two successive days are not vertically above one another, and the 

 •want of agreement of the time lines for two or more curves is apt to be 

 confusing. 



