18 REPORT OF THE SECRETAEY. 



bers during the year, should form a part of this volume ; unfortu- 

 nately, however, the plates intended for its illustration were destroyed 

 in the fire, and its place in the volume has been supplied by the fourth 

 article, which has since been presented for publication. 



In the reports for the last three years an account has been given 

 of a series of papers containing the deductions from the magnetic ob- 

 servations at Girard College, Philadelphia, by Professor A. D. Bache, 

 Superintendent United States Coast Survey. The whole of this series 

 of papers was divided into four sections, each containing three parts. 

 The object of the whole series is to present the results deduced from 

 the changes observed in the direction and intensity of the magnetic 

 force of the earth as apparently affected by the position of the sun 

 and moon relative to the earth and to each other. 



The first section related to the disturbances in the line of the de- 

 clination, or of the fitful variation, as it is called, of the magnetic 

 needle, and to the regular variations of the declination. 



The second section related to the variation in the intensity of the 

 magnetic force of the earth, estimated in a horizontal direction. 



The third section related to the same force as estimated in a verti- 

 cal direction. 



The fourth section relates to the perturbations or fitful changes in 

 the direction and intensity of the total magnetic force of the earth 

 as estimated in the direction of the dipping needle. 



The first three of these sections have been described in previous 

 reports, and it now only remains to give an account of the fourth and 

 last. 



The data for the deductions given in this section are the quantities 

 observed in the variations of the horizontal and vertical components 

 of the magnetic force, expressed in minute scale divisions corrected 

 for progressive changes in the magnetism of the bars and for changes 

 due to temperature. The object of the investigation was to deter- 

 mine the law of the great disturbances to which the total intensity 

 and direction of the magnetic force of the earth is subjected. It is 

 well known that the intensity and direction of the magnetic force of 

 the earth do not remain the same from hour to hour, but are subject 

 to regular fluctuations connected with the day and the season, and 

 also to larger perturbations, which have until lately been considered 

 fitful, and have therefore received the name of magnetic storms. The 

 special object of investigation of the first part of the fourth series is 

 to ascertain the average character of the large disturbances, and to 



