On a disturbance of the earth's magnetism : in connexion with the appearance of an aurora borealis, as observed at Albany, April 19, 1831 / by Joseph Henry.

  • Henry, Joseph, 1797-1878.
Date:
[1832?]
Catalogue details

Licence: Public Domain Mark

Credit: On a disturbance of the earth's magnetism : in connexion with the appearance of an aurora borealis, as observed at Albany, April 19, 1831 / by Joseph Henry. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    volume of the American Journal of Science. One of these, it will be recollected, formerly belonged to Prof. Hansteen of Norway, and the other to Capt. Sabine. They were suspended, according to the method of Hansteen, in a small mahogany box, by a single fibre of raw silk. The box was furnished with a glass cover, and had a graduated arc of ivory on the bottom, to mark the amplitude of the vibrations: It had also two small circular windows, dia- metrically opposite to each other, through which the oscillations of the needle could be seen. In using this apparatus, the time of 300 vibrations was noted by a quarter second watch, well regulated to mean time ; a register being made at the end of every tenth vibration, and a mean de- duced from the whole, taken as the true time of the 300 vibrations. Experiments carefully made with this apparatus, were found sus- ceptible of considerable accuracy ; as the individual observations, after a small correction for temperature, give a result, except in a few instances, differing from the mean of a number made under similar circumstances, by a quantity not greater than one part in nearly a thousand. The observations were repeated daily, when the weather would permit, from the latter part of September to the last of November, either at the hours of 12 at noon, or between 5 and 6 p. m.* I was always assisted in making them by the same person, my rela- tive, Mr. Stephen Alexander, to whose skill and experience I am much indebted for any accuracy they may possess. In April, 1831, a new series was commenced, to determine if the needles still indicated the same degree of magnetic intensity. No material difference was observed, except in the following in- stance, when a remarkable anomaly was exhibited. On the 19th of April, at 12 o'clock at noon, an observation was made with the Hansteen needle, the result of which differed only the fractional part of a second from the usual mean rate of this needle. At 6 o'clock p. m. the same day, another observation was made with the same needle, and apparently under the same cir- cumstances ; but a remarkable change was now observed in the time of its making 300 vibrations, indicating a great increase in the magnetic intensity of the earth. It was at first supposed that the * Theie times were chosen only on account of being most convenient.