2Y2 BULLETIN OF THE 



wei'e continued daily fov two months.* In April, 1831, a second 

 series of observations was commenced ; in the course of whiuh 

 his attention was attracted by a great disturbance of the needle 

 during the time of a conspicuous " aurora" on the 19th of April, 

 1831. At noon of the 19th the oscillations were found to be 

 perfectly'" accordant with previous ones, but at 6 o'clock P. M. a 

 remarkable increase of magnetic intensity was indicated. At 10 

 o'clock of the same evening, during the most active manifestation 

 of the aurora, the oscillations of the needle were again examined. 

 " Instead of still indicating as at 6 o'clock an uncommonly high 

 degree of magnetic intensity, it now showed an intensity con- 

 siderably lower than usual." Thus, designating the normal 

 intensity at the place as unity, at 6 o'clock it had increased to 

 1.024, and at 10 o'clock had subsided to 0.993, which according 

 to Hansteen's observations is the usual relation of magnetic dis- 

 turbance by an aurora. f An account of these results was com- 

 municated by Henry to the Albany Institute, January 26, 1832; 

 ami was also published in the Report of the Regents of the JS'ew 

 York University. A little more than a month later (to v>;it on 

 March 6, 1832,) he had been able to collate the various published 

 accounts of this aurora; and he learned "the fact of a dis- 

 turbance of terrestrial magnetism being observed by Mr. Christie 

 in England on the same evening, and at nearly the same time 

 the disturbance was witnessed in Albany, and that too in con- 

 nection with the appearance of an aurora." This circumstance 

 led him to make a careful comparison of the notices of auroral 

 displays given in the meteorological reports in the Annals of 

 Philosophy for 1830 and 1831, with those of the Reports of the 

 New York Regents for the same period. " By inspecting these 

 two publications it was seen that from April, 1830, to April, 1831, 



* The needles employei in these observations were a couple received 

 by Professor Reiiwiuk from Capt. Sabine, — one of which bad belonged to 

 Piof. Hansteen of Norway. " They were suspended according to the 

 method of Hansteen in a small mahogany box, by a single iSbre of raw 

 silk. The bos was furnished with a glass cover, and bad a graduated 

 arc of ivory on the bottom to mark the amplitude of the vibrations. In 

 using this apparatus, the time of three hundred vibrations was noted by 

 a quarter-second watch, well regulated to mean time ; a register being 

 ina<le at the enii of every tenth vibration, and a mean deduced from the 

 whole, taken as the true time of the three liundred vibrations.^ Experi- 

 ments carefully made with this apparatus were found susceptible of con- 

 siderable aci-uracy : " the individual observations not differing from the 

 mean number, ordinarily more than one-thousandth. (Sillimau's Am. 

 Jour. Sci. April, J 832, vol. xxii. p. 145.) 



f Prof. Hansteen lias remarked that "A short time before the aurora 

 borealis appears, the intensity ot the magnetism of the earth is apt to rise 

 to an uncommon heigl.t ; but so soon as the aurora borealis begins, in 

 proportion as its force increases, the intensity of the magnetism of the 

 earth decreases, recovering its former strength by degrees, often not till 

 the end of twenty-lour hours." {Edinburgh Philosoph. Jour. Jan. 1825, 

 vol, xii. p. 91.) 



46 



