TREFACE. 



In 1856, Baron von Miiller, of Germany, undertook an exploration in reference 

 to the natural history of Mexico, and proposed to the Smithsonian Institution to 

 make in its behalf a magnetic survey of the same country. 



The offer having been accepted by the Institution, an appropriation was made 

 to pay a portion of the salary of Mr. Sonntag, the assistant of Baron von Miiller ; 

 and the magnetic instruments were intrusted to him which had previously been 

 lent to Dr. Kane, and used by Mr. Sonntag himself, as one of the assistants of this 

 lamented explorer in his last expedition. 



Several records of observations, made at various places in Mexico, were at 

 different times forwarded by Baron von Miiller previous to his return to Germany, 

 after which the Institution was informed by him that the instruments had been 

 captured and destroyed by robbers. 



No final report having been furnished by Baron von Miiller, the crude obser- 

 vations made previous to the loss of the instruments have been reduced for 

 publication at the expense of the Institution, by Mr. Sonntag, and the results given 

 in the following pages. 



Appended to the magnetical observations will be found a series of notes by 

 Mr. Sonntag, upon the volcano of Popocatepetl and its vicinity. 



The present memoir was critically examined by Mr. C. A. Schott, of the Coast 



Survey, and has been submitted to Professors Guyot and Alexander, of Princeton, 



as a commission of reference. 



Joseph Henry, 



Secretary S. I. 



