20 EEPOET OF THE SECRETAET. 



The 6tli and last part of this interesting series relates to the lunar 

 influence on the horizontal magnetic force. 



Each observation, after being corrected for temperature and for the 

 progressive changes in the earth's magnetism, was marked with the 

 corresponding lunar hours, reckoning from the passage of the moon 

 over the meridian. The difference was then taken between the posi- 

 tion of the magnetic bar, as calculated from all the hours and 

 that found by observation at each lunar hour, and the difference 

 was considered as the effect of the magnetism of the moon on the 

 earth. 



From these and similar observations, it appears that the moon 

 exerts a magnetic influence on the earth. The principal magnetic 

 effect takes place 2 h. 52 m. after the upper passage of the moon 

 over the meridian: secondary effect, 1 h. 7 m. ; lower passage, princi- 

 pal minimum, at G h. 41": lower passage, secondary minimum, at 8 

 h. 19 m. after upper. 



Another paper contains an abstract of the observations and results, 

 with the discussion of a detailed magnetic survey of Pennsylvania and 

 part of the adjacent States of New York, Ohio, and Maryland, 

 originally made by Professor Bache, and resurveyed by Mr. Charles 

 A. Schott. 



In the study of terrestrial magnetism, two objects are essentially 

 necessary: first, the direction and intensity of the magnetic force of 

 the earth at a given epoch; and secondly, the variation in these ele- 

 ments after a known interval. 



In 1840, 1841, and 1843, Professor Bache made a series of obser- 

 vations to determine the magnetic elements at a number of prominent 

 places in Pennsylvania, New York, Ohio, Maryland, and Canada. The 

 Avhole number of stations at which he ascertained the declination or 

 variation was 16, and of those where the dip and intensity was ob- 

 served 48. 



Last summer Mr. C. A. Schott, an assistant in the coast survey, 

 visited six of the stations at which Professor Bache had previously 

 observed the magnetic elements, and by carefully redetermining 

 them obtained the data for calculating the secular changes which 

 had taken place during an interval of about twenty-years. The ob- 

 servations originally made by Professor Bache, and also the late ones 

 by Mr. Schott, were obtained with instruments of great precision, 

 and will always furnish the elements of future comparison for the 

 study of the changes which the magnetic force of the earth undergoes 



