AT A' AN RENSSELAER U ARBOR. 



n 



Mean Diurnal Inequality of Declination during Seventeen Days in January, Pfbruary, 

 AND March, 1854, at Van Rensselaer Harbor, and at Greenwich during the same days; 

 expressed in Minutes op Arc. 



A negative sign indicates a deflection to the east, a positive one a deflection to 

 the west of the mean position. 



The diurnal inequality at the two stations presents in general the same charac- 

 teristic features, namely, the principal deflection to the west shortly after noon, 

 and the opposite eastern position about midnight ; in regard to the diurnal ine- 

 quality, therefore, the motion of the magnet at Van Rensselaer Harbor follows in 

 general the same law as recognized in lower geographical latitudes. 



The extreme westerly position is attained at noon ; after this hour the westerly 

 declination diminishes gradually, with an exception of a period of opposite motion 

 of very limited range between the hours of four and five. The easterly extreme 

 is reached two hours after midnight. Whether the small irregularity just noticed, 

 producing apparently a secondary minimum and maximum, is real or only caused 

 by the accidental deviations of the few observations under discussion, it is not easy ' 

 to decide with certainty. The motion from 14 hours to 24 hours is performed with 

 great uniformity. Thus, while the diurnal motion agrees with that observed at 

 Lake Athabasca, Fort Simpson, Sitka, Toronto, etc., it shows no trace of that 

 marked deviation observed at Reikiavik, in Iceland, or at Fort Confidence. In 

 1824 (June), at the Whalefish islands the maximum westerly deviation happened 

 about a quarter past one o'clock P. M. ; the time of the maximum eastern deflection 

 was not determined. At Port Bowen the maximum westerly variation appears to 

 have occurred between the hours of 10 A. M. and 1 P. M., the mean result being: 

 ll"" 49™; the greatest deflection of the north end of the needle to the eastward 

 took place between 8 P. M. and 2 A. M., the mean hour being 10 P. M. These 

 observations were made during January, February, March, and April, 1825. 



The range of the mean diurnal inequality is 1° 06'.8, when it is at Greenwich 

 during the same time 10'.9. 



Analysis of Disturbances of the Declination. — The declination at the commence- 

 ment and end of the observations appears to have remained nearly the same ; the 

 daily and monthly means indicate at first a gradual decrease of westerly declination, 

 which motion, however, is speedily overcome in the month of March. No further 

 attention need be paid to this circumstance in the following discussion of the dis- 

 turbances, and of their effect upon the diurnal inequality. 



The mean disturbance for each of the 24 hours has been obtained by comparing 

 the monthly mean with each hourly reading; let A equal this difference, n the 

 3 



