On the occasion of the Aurora seen by me at Guildown, 4th February, 1872, rain fell immediately succeeding the formation of the corona.
The falling of rain as an immediate sequence of an Aurora seems, however, to be rather the exception than the rule; but possibly this may vary with the character of the Aurora itself—whether it be of the crimson class, passing into cloud and accompanied with much electric disturbance, or of the more quiescent white.
A falling barometer observed to follow Auroræ.
A falling barometer following a display of Auroræ has been noticed by Sir John Franklin and others; and in some cases (notably one in Sicily before referred to) storms and floods have accompanied this.
Professor Christison’s observations.
In a paper read before the Royal Society of Edinburgh in 1868, Prof. Christison mentioned, as a fact of importance to agriculturists, that the first great Aurora after autumn is well advanced, and following a period of fine weather, is a sign of a great storm of rain and wind in the forenoon of the second day afterwards.
Mr. C. L. Prince, in his ‘Climate of Uckfield,’ p. 218, remarks that displays of Auroræ are almost invariably followed by very stormy weather, after an interval of from 10 to 14 days.
Aurora and the Magnetic Needle.
Aurora and the magnetic needle. Sir John Franklin’s observations. Motion communicated to the needle was neither sudden nor vibratory. Return of needle to its former position very gradual. Different positions of the Aurora had considerable influence on the direction of the needle. Needle disturbed when Aurora not visible. Quiescent yellow Aurora produced no perceptible effect on needle. Return of needle more speedy after formation of a second arch. Slow when disturbance was considerable.
Sir John Franklin, in his ‘Narrative’ (before referred to), gives Lieutenant Robert Hood, R.N., the credit of being “the first who satisfactorily proved, by his observations at Cumberland House (before mentioned), the important fact of the action of the Aurora upon the compass-needle,” and also “to have proved the Aurora to be an electrical phenomenon, or at least that it induces a certain unusual state of electricity in the atmosphere.” Sir John Franklin then mentions that the motion communicated to the needle was neither sudden nor vibratory. Sometimes it was simultaneous with the formation of arches, prolongation of beams, or certain other changes of form or of activity of the Aurora. But generally the effect of these phenomena upon the needle was not visible immediately; but in about half an hour or an hour the needle had obtained its maximum of deviation. From this its return to its former position was very gradual, seldom regaining it before the following morning, and frequently not until the afternoon, unless it was expedited by another arch of the Aurora operating in a direction different from the former one. The magnetic needle in the open air was disturbed by the Aurora whenever it approached the zenith. Its motion was not vibratory (as observed by Mr. Dalton), perhaps owing to the weight of the card. It moved slowly to the E. or W. of the magnetic meridian, and seldom recovered its original direction in less than eight or nine hours. The greatest extent of its aberration was 45´. The arches of the Aurora were remarked commonly to traverse the sky nearly at right angles to the magnetic meridian; but deviation was not rare, and it was considered that the different positions of the Aurora had considerable influence on the direction of the needle. When an arch was nearly at right angles to the magnetic meridian, the motion of the needle was towards the W. This motion was greater when the extremity of the arch approached from the west towards the magnetic north. A westerly motion also took place when the extremity of an arch was in the true north, or about 36° to the west of the magnetic north. The motion of the needle was towards the east when the same end of an arch originated to the southward of the magnetic west, and when of course its opposite extremity approached nearer to the magnetic north. In one case only a complete arch was formed in the magnetic meridian. In another the beam shot up from the magnetic north to the zenith. In both these cases the needle moved towards the west. The needle was most disturbed on February 13, 1821, at a time when an Aurora was distinctly seen passing between a stratum of clouds and the earth. Sometimes the needle deviated though no Aurora was visible; but it was uncertain whether there might not have been a concealed Aurora at the time. Clouds were sometimes observed during the day to assume the form of the Aurora, and deviations of the needle were occasionally remarked at such times. An Aurora sometimes approached the zenith without producing any change of position of the needle; while at other times a considerable alteration took place, though the beams or arches did not come near the zenith. The Aurora was frequently seen without producing a perceptible effect on the needle. At such times it was generally an arch or a horizontal stream of dense yellowish light with little or no internal motion. The disturbance of the needle was not always proportionate to the agitation of the Aurora, but was always greater when the quick motion and vivid light were observed to take place in a hazy atmosphere. In a few instances the needle commenced at the instant a beam started from the horizon upwards; and its return was according to circumstances. If an arch formed immediately afterwards, having its extremities placed on opposite sides of the magnetic north and south to the former one, the return of the needle was more speedy, and it generally went beyond the point from which it first started. When the disturbance was considerable, it seldom regained its usual position before 3 or 4 P.M. on the following day. On one occasion only the needle had a quick vibratory motion (between 343° 50´ and 344° 40´). The disturbance produced by the Aurora was so great that no accurate deductions as to diurnal variation could be made.