INTRODUCTION. 



IN co-operation with the scheme adopted at the British colonial observatories, 

 a series of magnetic and meteorological observations were made at the Girard 

 College magnetic observatory, in Philadelphia, with instruments purchased under 

 the direction of the trustees of the college, the observations being made under the 

 patronage of the American Philosophical Society, and finally completed for the use 

 of the topographical bureau of the War Department. 1 These observations were 

 made under my immediate direction, and were afterwards left under my general 

 superintendence. The series commenced in May, 1840, and, with short inter- 

 ruptions, terminated in June, 1845, thus furnishing a five years' series of magnetic 

 observations, taken bi-hourly up to October, 1843, and after that date hourly. The 

 readings of each magnetic element were united into mean values, arranged accord- 

 ing to hours of the day and days of the month and annual values, and presented 

 graphically, under my direction, by Joseph S. Ruth, Esq., who had taken part in 

 the observations, and who was at that time employed in the Coast Survey. As, 

 owing to other laborious duties, the record could not then be submitted to a com- 

 plete reduction and discussion, I have resumed the subject, with the aid of Charles 

 A. Schott, Esq., assistant in the Coast Survey, by whom, under my immediate 

 direction, the discussions contained in this paper have been made and prepared for 

 publication. It is proper to state that this work has been performed out of office 

 hours by Mr. Schott, as my assistant in this special matter, and at my own expense. 



Although the magnetic observatories furnished by their judicious geographical 

 location, a basis for the generalization of their results, it is, nevertheless, desirable 

 to combine other results with them as confirmations, or as corrections. In the 

 investigation of the disturbance law at Point Barrow, as compared with the same 

 at Toronto, a very remarkable mutual relation was developed, and further exami- 

 nation may bring to light other dependencies of a mutual character. 



According to the latest determination, the position of the Girard College observa- 

 tory is in latitude 39 58' 23" (north), and in longitude 75 10' 05" = 5" 00 m 40'.3 



* See " Observations at the magnetic and meteorological observatory at the Girard College, Philadel- 

 phia, made under the direction of A. D. Bache, LL. D., and with funds supplied by the members of the 

 American Philosophical Society and by the Topographical Bureau of the United States, 1840 to 1845. 

 Printed by order of the Senate of the United States, and under the direction of the Topographical 

 Bureau, second session of the twenty-ninth Congress. Washington, D. C., 1847." Three volumes 

 record and one volume plates. 

 1 



