Miscellanies. 211 
Tt is well known that the magnetic declination, at a given place, is su 
ject to frequent and irregular variations, and that sabvcaieaalag Sache 
occur, at the same instant of time, at very distant places. The first re- 
cognition of this remarkable phenomenon seems to have been made by 
Arago, while comparing the observations of declination made by himself, 
at Paris, in the year 1818, with the contemporaneous observations of M. 
Kupffer, at Casan. Not long after, the subject underwent a fuller inves- 
tigation in the hands of Humboldt; and, in the year 1827, an extensive 
system of simultaneous observations was organized by that illustrious 
philosopher for the purpose of elucidating it. At length, in 1834, it was 
taken up by Gauss, and received a much greater development. Gauss 
discovered that the irregular changes of the declination were of continual 
occurrence; and that the synchronism, which had been previously ob- 
served only in the larger changes, extended to the minutest movements. 
Ta order to investigate the law of these synchronous changes, and the 
locality and other circumstances of the acting forces, Gauss arranged the 
extensive plan of simultaneous observations at short intervals, which has 
been already four years in operation, and i in which almost every country 
in Europe has been represented by some one or more observers 
The rapidity with which these changes thus appeared to Gita each 
other, held out the hope that they might be employed in determining dif 
ferences of longitude ; and it only remained to ascertain, for that 
whether variations so inconsiderable in magnitude ‘cotresponded at great 
distances. If such should prove to be the fact, it would be only necessary 
to project on a large scale the results of the observations made about the 
same absolute time at the two stations, and to compare the times of the 
corresponding maxima and minima. In the observations already referred 
to, the maxima and minima succeeded each other (as has been said) at 
intervals of about forty seconds, and the epoch of their occurrence was 
Probably known to six or seven seconds. By shortening still further the 
interval: of observation, it is manifest that this error may be much dimin- 
The corresponding error of the difference of longitude resulting 
from a single comparison (supposing the probable error of epoch to be the 
same at the two places) will be greater in the ratio of 1/2 to 1; but this 
error, owing to the multitude of the maxima and minima cotupared; must 
necessarily be greatly reduced in the final mean. 
In order to put this question to the test on the largest scale, it was 
agreed between Mr. Bache and Mr. Lloyd, to make a series of- corres- 
ponding observations in Philadelphia and in Dublin. Some difficulties 
occurred in concerting a plan, and Prof. Bache underwent, in one in- 
stance, the labor ofan extensive series of observations, without any coun- 
terpart in Dublin. At length, however, it was agreed to observe during 
the week commencing the 11th of November, 1839; the observations 
being taken during two hours on each day—namely, from 12 to '?-@., 
and from 8 to 9 P. M., Greenwich mean time. 
