beheld by Farquharson, of Scotland, was computed to be as low as 4300 feet. Franklin thus remarks: "The fact that the aurora exists at a less height than that of dense clouds, was evinced at Fort Enterprise, on two or three occasions, during the night of the 13th of February, 1821, and particularly about midnight, when a brilliant mass of light, variegated with the prismatic colors, passed between a uniformly steady, dense cloud and the earth. In its progress, that portion of the cloud which the stream of light covered was completely concealed until the coruscation had passed over it, when it appeared as before." 573. A similar, but more extraordinary phenomenon, which occurred during his third Arctic voyage, is thus related by Capt. Parry. "While Lieutenants Sherer, Ross, and myself were admiring the extreme beauty of the northern lights, we all simultaneously uttered an exclamation of surprise, at seeing a bright ray of the aurora shoot suddenly downward from the general mass of light, and between us and the land, which was there distant only three thousand yards. I have no doubt, that the ray of light actually passed within that dis tance of us." 574. SOUNDS ATTENDING THE AURORA. It has been asserted, that the aurora is sometimes accompanied by a noise like the rustling of silk, or the sound of a fire when excited by the wind; but much difference of opinion has arisen upon this point. Those who are incredulous in this particular, affirm that the noise in question may be nothing more than the murmur of the ocean, or of the forest; the rustling of the snow as it is driven by the wind, or the crackling sound that arises from its freezing; all which, it is said, might be easily attributed to the aurora, when the mind is excited by the wondrous spectacle, and susceptible to every illusion -the splendors that burst upon the sight, and the sounds. which strike the ear being then referred to the same origin. State the facts showing that the aurora is at times very near the earth. Give the facts respecting the sounds attending the aurora. 575. Scoresby, Richardson, Franklin, Parry and Hood, during their Polar expeditions, never heard any sound which they considered as proceeding undeniably from the northern lights, though hissing noises were heard during the auroral displays which were attributed by them to one or more of the preceding causes. These observers do not, however, deny, that at times audible sounds proceed from the aurora, and even express such a belief, founded upon the concurrent testimony of the natives of the arctic climes. 576. Credible observers in Iceland, Siberia, and Scandinavia, have never heard these singular sounds; nor were they perceived by the French scientific expedition, which wintered at Bossekop, in 1838-39; but Hansteen claims to have established their existence from a series of observations in the high northern latitudes. Upon this subject, Simpson thus remarks in his Northern Discoveries when speaking of a brilliant aurora seen by his attendant, at Fort Confidence, on the 5th of March, 1839, "The aurora seemed to ascend and descend, accompanied by an audible sound resembling the rustling of silk. This lasted about ten minutes, when the whole phenomenon suddenly rose upwards, and its splendor was gone. Ritch is an intelligent and credible. person, and on questioning him closely, he assured me that he had perfectly distinguished the sound of the aurora from that produced by the freezing of the breath, for the temperature was forty-four degrees below zero. I can therefore no longer entertain any doubt of a fact uniformly asserted by the natives, and insisted on by my friend Mr. Dease, and by many of the oldest residents of the fur countries, though I have not had the good fortune to hear it myself." 577. TIME. The appearance of the northern lights is not confined to any particular hour of the night, a fact which is fully proved by the circumstance that the same display is frequently witnessed at places widely differing in longitude. Thus, if the aurora extends Does the aurora appear at any particular hour? from Boston, Mass., to Berlin, in Germany, and is bu held simultaneously at these cities, the difference in the reckoning of time will be nearly five hours and a half (C. 939). 578. There is much reason for believing that the aurora sometimes occurs during the day, though rendered invisible by the presence of the sun. Richardson perceived at Bear Lake, the motion of the aurora before the entire disappearance of twilight, and even during the day he discerned clouds, arranged in columns and arches, resembling those of the northern lights. Besides, as we shall show hereafter, a brilliant display of this phenomenon. is always accompanied by a greater or less disturbance of the magnetic-needle, (C. 997,) and as these disturbances take place in the day as well as in the night, it is reasonable to infer that they are caused by the presence of an invisible aurora. 579. FREQUENCY. This phenomenon is more frequently seen in winter than in summer; we must not, however, hastily conclude from this circumstance, that the number of auroras during the former season is actually greater, for the increased length of the nights during the winter would enable us then to see more displays of the northern light, even if the times of its occurrence were equally distributed throughout the year. About the period of the equinoxes they also appear to be more frequent. These facts are shown from the following table of Kaemtz, which gives the number of auroras that have been seen in each month. Why is it supposed sometimes to occur in the day? What is said respecting the frequency of its appearance in winter and summer? Recite the table. 580. In addition to this annual variation, there appears to be another which extends through a considerable number of years, but of which very little is known. Thus, from 1707 to 1752, the northern lights became more and more frequent; but after the latter date, a period of twenty years occurred, in which they diminished in number. An increase in their frequency began in 1820, and since that period many magnificent displays have been witnessed. The number observed for the last ten years, at New Haven, Ct., by Mr. E. C. Herrick, is shown in the following table. Between the 12th of September, 1838, and the 18th of April, 1839, no less than one hundred and forty-three distinct auroras were seen by the French observers at Bossekop. They were most frequent at the period when the sun was below the horizon, viz.: from the 17th of November to the 25th of January. During this night of ten weeks, sixty-four auroras were visible. 581. DISTURBANCE OF THE MAGNETIC-Needle. During the prevalence of the aurora, the compass-needle, instead of remaining motionless, in the magnetic meridian, is often much disturbed. Sometimes it is deflected toward the east several minutes and even degrees; then Is there any other probable variation? Recite the table. What is said respecting the disturbance of the compass-needle ? it is agitated, and returns either slowly or rapidly, to the meridian, which it passes at times and moves toward the west. These deviations are as changeable as the phenomenon itself. When the arch is motionless the needle is quiet; its disturbance commences when the streamers begin to play. 582. Franklin observed at Fort Enterprise, that the disturbance of the needle was simultaneous with some change in the form or action of the northern lights, and that after being deflected it returned to its former position very gradually, not resuming it before the following morning, and sometimes even not before noon. Moreover when the auroral arch was either at right angles to the meridian, or its western extremity north of west, the needle was deflected toward the west; but if its western extremity was south of west, the needle moved toward the east. During the aurora of November 14th, 1837, the en tire range of the needle at New Haven, was observed by Messrs. Herrick and Haile to be nearly six degrees. It was not until the morning of the next day, between seven and nine o'clock, that the needle was at rest in its usual position. 583. This effect upon the magnetic needle during the prevalence of the northern lights, was noticed for the first time by Celsius and Hiorter, at Upsal, on the 1st of March, 1741. 584. It is asserted by Wilke, that when the aurora appears in great splendor, the position of the dippingneedle is as variable as that of the compass-needle; the former rising and falling with the northern crown. Hansteen has also observed, that the dipping-needle descends very much below its usual position before the aurora is visible; but that after the display commences it begins to rise: and more rapidly in proportion to its brightness. The needle then slowly resumes its original position, which it frequently does not attain unul How great was its range at New Haven, November 14th, 1837 ? |