Disturbance of the Earth’s Magnetism. 145 
may be useful, either in directing the attention of observers in this 
country to the subject, or in corroborating similar observations made 
in other quarters of the globe. 
In September, 1830, I commenced a series of observations, for 
Professor Renwick, of Columbia College, to determine the mag- 
netic intensity at Albany. In the course of these, I unexpectedly 
witnessed a disturbance of the magnetism of the earth, in connexion 
with an appearance of an aurora, which on some accounts appears 
interesting. 
The needles used in these observations, were those mentioned in 
Capt. Sabine’s letter to Prof. Renwick, published in the 17th vol- 
ume 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 are of ivory on the bottom, to mark the amplitude 
of the vibrations. It had also two small circular windows, diamet- 
rically opposite to each other, through which the oscillations of the 
needle could be seen. 
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 made at the end of every tenth vibration, and a mean 
deduced from the whole, taken as the true time of the three hundred 
vibrations. Experiments carefully made with this apparatus, were 
found susceptible of considerable accuracy; as the individual ob- 
servations, after a small correction for temperature, give a result, ex- 
cept 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 6p.m.* [ 
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. 
» These times were chosen only on account of being most convenient. 
Vou.—XXil. No 1. 19 
