28 REPORT OF THE SECRETARY. 



the summer months, and the others those for the winter months. 

 After being subjected to a similar process of reduction, it was found 

 from these that the lunar variation is much smaller in amplitude in 

 winter than in summer, and also that the maxima and minima occur 

 earlier in the former than in the latter season, the winter curve pre- 

 ceding the summer curve by about an hour and three quarters. 



Professor Bache next proceeded to ascertain whether the phases, 

 declination, or parallax of the moon have any sensible effect on the 

 magnetic variation. Dr. Kreil, from the discussion of ten years' ob- 

 servation at Prague, concluded that there was no specific change in the 

 position of the magnet depending on the moon's phases or parallax, 

 but that the variation was sensibly greater when the moon was at its 

 greatest northern declination. On the contrary, Mr. Brown, from a 

 much shorter series of observations in India, inferred that there was 

 a minimum of variation two days after the full moon. To investigate 

 these points, the lunar variation for the days of full and new moon, 

 and for two succeeding days, were compared with the average monthly 

 variation ; the results indicate that the north . end of the magnet is 

 deflected six seconds to the westward at full moon, and as much to the 

 eastward on the day of new moon. This quantity is not much beyond 

 the probable error of observation, but a more definite result could hardly 

 be expected from a series extending over but five years. The period 

 of the observations is also too short to exhibit any definite variation 

 depending on the moon's greatest northern or southern declination, 

 and the same remark may be applied to the effect of the varying dis- 

 tance of the moon. Professor. Bache proposes, in another paper, to 

 extend the discussion to the moon's influence on the variation in the 

 intensity of the magnetic force of the earth. 



I neglected to mention in the last report that, besides the magnetic 

 observations made by Professor Bache in cooperation with the system 

 inaugurated by the British association, two other series were carried 

 on simultaneously- — one in the city of Washington, by Lieutenant 

 G-illiss, of the United States Navy, and the other by Professor Bond, 

 of Harvard University. The observations of Lieutenant Gilliss were 

 made once in two hours with a bar eleven inches long, observed with a 

 micrometer microscope reading to seconds of arc, and were continued 

 from July 7, 1840, to June 30, 1842, a period of two years. Beside 

 the bi-hourly series, another was made on term days, viz : on the 23d 

 and 24th of each month, from September, 1840, to" June, 1842, in 

 which the position of the needle was recorded at intervals of every 

 five minutes. Professor Bond's observations at Cambridge extended 



