AT CO-OPEEATING STATIONS. 



157 



61. Plate XIX refers to a storm recorded on August 25-26, 1903. It shows the D, H, and V curves 

 for Christchurch, and the H curves for Colaba and Falmouth. The largest movements did not occur until 

 nearly 9 hours after the commencement, and to bring the principal part and the beginning within the 

 range of one plate the trace is omitted from about 3.30 to 6.0 a.m. 



The Falmouth and Colaba traces show a sudden commencement. At Falmouth and Kew the 

 commencement, like that of April 5, 1906, showed a distinct double movement, a very small diminution in 

 H being followed by a large increase. The Declination curves at these two stations also show a double 

 movement, a small easterly movement being followed by a somewhat larger but still small westerly one. 

 The double movement occupied from 10.57 to 11.3 p.m. on the 25th, the to-and-fro movements 

 occupying about equal times. 



The Colaba H curve also shows at least a suggestion of a double movement, an increase of some size 

 following a very small decrease, the two corresponding in time to the corresponding changes at Falmouth. 

 The changes in the Colaba D and V curves are very small. 



At Christchurch there was also a sudden commencement, though it is relatively inconspicuous. It is 

 best seen in the D curve, where there was first a small movement to the east (i.e. down the sheet), from 

 about 10.57 to 11.0 p.m., followed by a larger movement of similar duration to the west. Whether 

 there was a double movement on the Christchurch V curve it is difficult to say, but there is at least an 

 indication of a movement down the sheet, preceding the one distinctly visible up the sheet. There is also 

 a distinct oscillation on the H curve, corresponding in time to that in the D curve. This was preceded by 

 various small movements, which presumably were from a different source. 



The results obtained from the measurements of the double movement were as follows : 



The increments AH, &c., here, as elsewhere, measure the dianges during the interval specified. Thus at 

 Kew, on the occasion in question, H fell Gy between 10.57 and 11.0 p.m., and then rose 34y between 1 1.0 and 

 11.3p.m. At 11.3p.m. H was thus greater by 28y than when the storm commenced. The change 

 between 11.0 and 11.3 p.m. was very probably in part a recovery from that experienced between 10.57 

 and 11.0 p.m., but that is purely a matter of surmise ; thus in the calculations subsequently made the 

 movement from 11.0 to 11.3 p.m. is treated as an absolutely independent one. 



62. Plates XX and XXI relate to a storm of considerable size which occurred on December 13, 1903. At 

 Falmouth, as Plate XX shows, a sudden commencement is distinctly visible in the D curve. There was a 

 distinct double movement, first to the east from about 0.29 to 0.30 p.m., then to the west from 0.30 to 

 0.33 p.m., the westerly movement being considerably the larger. In the case of the H curve there were 

 at the commencement several to-and-fro movements, of too small an amplitude to show details clearly. 

 The principal part of the disturbance occurred from 6 to 10 hours after the commencement. 



Plate XXI shows the changes in the Horizontal Force at Colaba and Mauritius corresponding to those at 

 Falmouth. Both curves show a very distinct sudden commencement, the initial movement lasting from 

 about 0.28 to 0.35 p.m. II was falling at both stations at the time when the disturbance began (at 

 Mauritius II increases down the sheet), and it is not absolutely clear that the first sudden change may 

 not have been a further decrease, but if so it was too small and instantaneous to be clearly visible. The 

 increase, on the other hand, is very decided. Following this there was a large but wonderfully uniform 

 fall in II, continuing for about six hours, in the course of which a decrease of over 150y was experienced both 

 at Colaba and Mauritius. At Falmouth the synchronous change in II was also on the whole a decrease, 



