Miscellanies. 211 



It is well known that the magnetic declination, at a given place, is sub- 

 ject to frequent and irregular variations, and that corresponding changes 

 occur, at the same instant oT 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. 

 Kupflfer, 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 cormnnal 

 occurrence ; and that the synchronism, which had been previously ob- 

 served only in the larger changes, extended to the minutest movements. 

 In order to investigate the law of these synchronous changes, and the 

 iocality 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 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 follow 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 purpose, 

 whether variations so inconsiderable in magnitude corresponded 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- 

 ished. 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 v/2 to 1 ; but this 

 error, owing to the multitude of the maxima and minima compared, 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 of an 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 1 P. M., 

 and from 8 to 9 P. M., Greenwich mean time. 



