PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. 



371 



THE PERTURBATIONS OF THE 15th JULY, 1883. 



(PI. XXIX). 



87. As the curves show, the storms occurring on the above date, especially those in the polar 

 regions, are exceedingly characteristic and well defined, and of considerable power. 



We have previously described principally magnetic storms that occurred in the winter, and two 

 or three perturbations about the spring equinox. Special interest will therefore attach to a case of a 

 magnetic storm occurring near the summer solstice, and the storm now to be described is a good example 

 of just such a storm. 



It may at the outset seem very unlikely that the main features in the occurrence and course of 

 the perturbations should change character; indeed one would rather expect to find the same principal 

 features, while the details might possibly exhibit more peculiar conditions. 



We will now go through the various phases, and see how well these assumptions are confirmed. 



We may consider the interval from 6' 1 to io'' as a first section, for during that time there occur 

 at several places, as the curves show, perturbations that are all comparatively slight, but sometimes 

 very well defined. The most powerful forces occur at Fort Rae, where the perturbation is a series of 

 brief impulses taking place at about 7 h 30"", 8 h 20, and from g h to g h 2o m . 



The deflections in the district Fort Conger to Cape Thordsen are particularly characteristic, and 

 the time of their commencement there is a little earlier than in the perturbation at Fort Rae. 



At Bossekop and the southern European stations, disturbances are only sometimes noticeable, and 

 the deflections are as a rule too small to be taken out. 



It may here be worth while pointing out one circumstance connected with this first perturbation, 

 namely, that there is at the same time a deflection in one of the earth-current components at Pawlowsk, 

 which exhibits a remarkable resemblance to the deflections in the magnetic curves to the north. Whether 

 this is accidental, or whether a close connection between these phenomena exists, we will not attempt to 

 decide here. In this connection we will refer to a later chapter where the earth-currents are described. 



As the systems acting here are rather weak, the drawing of the corresponding current-arrows on 

 the charts will not give a much clearer idea of the perturbation-conditions than we obtain by the direct 

 consideration of the curves. We have not therefore drawn any chart for this period of the storm: Its 

 field of operations appears to be rather limited, and its occurrence more or less local in the north. 



At Fort Rae, where it is about midnight at this time, the storm is of the nature of a negative 

 polar storm; but nothing decided can be said as to what it may be at the other stations. 



After this slight, comparatively brief perturbation, a long period supervenes during which the 

 conditions are normal. 



At about 14'', however, powerful perturbations begin to develope all round the polar stations. In 

 the district Fort Rae, Uglaamie and Ssagastyr, an exceedingly characteristic, powerful negative polar storm 

 developes, which also seems to act with considerable strength at Kingua Fjord, judging from the deflec- 

 tions in the horizontal intensity. At the last-named place, the system appears to be a little earlier in its 

 occurrence than at Fort Rae. We must not, however conclude too much from the conditions in the 

 horizontal intensity alone, as the deflections in declination have a greater significance at Kingua Fjord 

 than at the other stations. 



A perturbing force in the horizontal intensity will thus here produce current-arrows directed more 

 or less north and south, while at the other stations the variations in the horizontal intensity will answer 

 to current-arrows pointing east and west. It is therefore best here to keep principally to the charts for 

 a general idea of the conditions. 



In the district Jan Mayen, Bossekop and Little Karmakul, on the other hand, a fairly powerful 



