REPORT OF THE SECRETARY. 21 



The whole discussion clearly indicates a law of recurrence in the 

 frequency of the large disturbances, although the period over which 

 the observations extend was not sufficient to determine the interval. 

 The observations, however, indicate with great precision the time of 

 the minimum, the rate of diminution as the disturbances diminish in 

 approaching this period, and their increase as they recede from it. 

 The minimum thus found, of frequency of large disturbances, occurred 

 in August, 1843. 



The establishment of the elements of a law of periodicity in rela- 

 tion to changes of the magnetic force which, from the time they were 

 first noticed until within a few years past, were regarded as entirely 

 irregular, is in its relation to terrestrial magnetism a fact of import- 

 ance; but the value of this is highly increased when it is found that 

 these disturbances are connected with changes in matter foreign to 

 cur earth. To realize this, we must refer to a series of persevering 

 observations made day by day for thirty years on the spots of the 

 sun, by an astronomer named Schwabe, in an obscure town of Ger- 

 many. This devotion to an apparently unfertile field of inquiry was 

 finally rewarded by the discovery that the spots on the sun's disc are 

 subject to a regular law of recurrence, and that they pass through the 

 phases of periods of greatest and least frequency in about eleven years; 

 but strange to say, it was afterwards announced by General Sabine that 

 the period of recurrence of large magnetic disturbances coincides 

 both in duration and its epoch of maximum with the period discovered 

 by Schwabe in reference to the solar spots; that is, that at the period 

 of greatest disturbances there occurs the maximum number of spots,, 

 and vice versa. The investigations of Professor Bache serve to 

 establish this conclusion, and to furnish additional elements for a more 

 accurate comparison. From these results it is clear that the sun 

 exerts an influence on the magnetism of the earth which depends on 

 the existing state of its own luminous atmosphere, affording another 

 example to be added to other illustrations of the same truth, that 

 scientific researches, if skilfully and perseveringly continued, will 

 always lead to valuable results, and often to those which could not 

 have been anticipated by any previous conceptions. 



The volumes of records of the Girard observations, which present 

 on casual examination immense series of tabulated figures in which 

 no law or regularity is observable, when scientifically studied and 

 properly interpreted, are thus found to yield truths of the highest 



