OF THE MAGNETIC DECLINATION, 



' The readings from June 1st to Jul}' 18th, 15 hours, on the College building scale, were converted 

 into observatory scale readings by subtracting 144'^.7 at division 628.8 of the old scale, and converting 

 the value of a division 0'.345 of the old into the corresponding reading for the value of a division 0'.453 

 of the new scale. The mean readings, thus corrected, of the first 18 days of July were then properly 

 combined with the mean of the remaining days of the month. 



' In the month of September, hour 8, the comparisons were made with the half monthly means, owing 

 to the rapid change of the readings. 



' On the 23d of November the inde.x of the declinometer bar shifted 10. .5 .scale divisions ; a correction 

 of +19''.5 has, therefore, been added to observations after this date, and, likewise, to all the readings of 

 the following month. 



* The corrections here given for referring the mean of the last seven months of the year to the mean 

 for the whole year, are derived from the normals of the following year 1841 by comparing the mean of 

 the same seven months with the annual mean of that year. Comparing the same months in the two 

 years the character of the changes appears to be about the same. 



= A further correction for change in the zero of the scale required to refer the readings of 1840 to the 

 readings of subsequent years. Owing to a rearrangement of the instruments on January 7, 1841, the 

 scale readings changed 112.8 divisions, and since 19.5 scale divisions had been added to the December 

 readiug.s, the resulting correction is the difference of the two, or +93''. 30. 



