386 M. A. Secchi on the Periodical Variations 



3. The discussion of the hypotheses hitherto propounded for 

 their explanation. 



I hope this memoir will not be unacceptable to my readers, as 

 the sources to which I have fortunately access, if not all that are 

 possible, are at least the best, and moreover are such as are not 

 at every one's command. 



At the present time, when observations and inquiries of all 

 kinds are so much multiplied, it is besides most desirable that 

 such classical works as have already been performed should be 

 widely known, that time and labour may not be lost by doing 

 again what has been already well done by others. 



Part I. — Brief historical Exposition of what has been under- 

 taken for the study of the Magnetic Variations. 



In the study of great problems in terrestrial physics such as 

 the present, the zeal and activity of a single observer are quite 

 insufficient, not only on account of the multitude of data required, 

 but also because simultaneous observations in many countries 

 are needed ; thus the aid of scientific societies, and sometimes 

 even the active cooperation of governments, becomes indis- 

 pensable. So long ago as 1761, the observations of Cassini in 

 France and Gilpin in England, repeated at Rome by P. Asclepi 

 in ] 762, had shown that the needle has a diurnal variation, and 

 the latter had even suspected an annual variation. In 1741, 

 Celsius in Sweden and Graham in London, by a series of con- 

 certed corresponding observations, discovered the simultaneity of 

 great magnetic perturbations at different points of the globe ; a 

 discovery which was afterwards forgotten, and was remade by the 

 observations, also simultaneous, of Arago in Paris and Kupffer at 

 Kasan in 1825. 



The illustrious Baron Alexander von Humboldt undertook 

 in 1806, at first alone and after wurds with the help of others, a 

 series of uninterrupted observations on certain days in the year. 

 They consisted in observing the needle every five minutes on the 

 days of the solstices and equinoxes. The apparatuses employed 

 by these first observers were different, — Humboldt used a mag- 

 netic coUimator of Prony, and Arago a variation needle of Gam- 

 bey ; but notwithstanding the anomalies which this circumstance 

 was likely to introduce, the simultaneity of the perturbations and 

 the parallelism of the movements of the needle were manifested 

 at Berlin, Paris, and in the Mines of Freyberg 66 metres deep. 

 Between 1832 and 1836, Gauss, having directed his learned re- 

 searches to the theory of terrestrial magnetism, furnished its 

 students with new methods and instruments of observation, and 

 thus introduced a new sera in the science. There was then orga- 

 nized a society of observers who undertook to make observations 



