XLIX. On the Periodical Variations of Terrestrial Magnetism, 

 By A. Secchi, Director of the Observatory of the CoUegio 

 Romano^. 



IN all physical problems it is from the intercomparison of phse- 

 nomena that great discoveries may be hoped for, and more 

 especially so when the cause of the phsenomena appears very ob- 

 scure and complicated. Such are the diurnal and annual variations 

 of the magnetic needle, which, being manifestly connected with the 

 sun's movements, have been attributed to that heavenly body, by 

 supposing it to act either directly as a magnetic body, or indi- 

 rectly through the variations which it produces in terrestrial 

 temperature. Dr. Miiller, in two memoirs published in the 

 Italian Ateneo (which he has kindly sent to me), has recently 

 examined the correspondence between the magnetic variations 

 and those of temperature, and has proposed new methods for 

 studying it in future. In the analogy found by him, and which 

 appears to me to have been carried too far, he has pei-haps been 

 led astray by an imperfect graphical method, and by paucity of 

 materials; those used by him being only the observations of 

 Arago and Quetelet, and of these only such as are quoted by 

 Kaemtz. It does not appear that he has attempted to discuss 

 the gigantic works, which have been executed in recent years in 

 so many and such well-equipped magnetic observatories scattered 

 over the globe, and more especially those established at the ex- 

 pense of the British Government, the observations of which have 

 been published and discussed by Colonel Sabine in nine large 

 volumes, published at the expense of the same Government ; for 

 Dr. Miiller says, in p. 2 of his memoii', ^Hhat in the absence of 

 long-continued observations and well- assured data, the results 

 hitherto obtained are but little satisfactory.'-' 



I have thought it useful to supply this omission, and at the 

 same time to complete the very imperfect information given re- 

 specting this branch of knowledge, even in the most esteemed 

 courses of instruction, by a brief exposition of what has been 

 hitherto done and discovered. While thus engaged, there pre- 

 sented itself to my apprehension a law of great simplicity, by 

 means of which all the facts are reduced to a very simple theory. 

 So much simplicity of law, in phsenomena apparently so irregular, 

 encouraged me to publish this memoir, which naturally divides 

 itself into three parts : — 



1. The historical exposition of what has been done towards 

 the study of the variations of terrestrial magnetism. 



2. The systematic exposition of the consequences deduced. 



* From the Corrispondenza Scientijica in Romtty July 15, 1854; com- 

 municated by Colonel Sabine. 



