AT TAN RENSSELAER HARBOR. 



19 



The month of February was, therefore, that of the maximum amount of disturb- 

 ance. At Lake Athabasca the greatest mean disturbance occurred in January 

 (from observations between October and February inclusive). At Toronto,^ on the 

 contrary, the months of January and June are those of least disturbance. It is 

 quite possible that at Van Rensselaer the above values are surpassed in other 

 months of the year, yet relatively February contains the greatest mean disturbance 

 during the period of observations. 



Hitherto the recognition and separation of the disturbed observations have been 

 effected by an arbitrary process of fixing upon a certain deviation from the mean 

 as the greatest allowable departure, and regarding all observations beyond this 

 limit as disturbances. In the present case, I have sought to introduce a more 

 definite idea by the application of Pierce's criterion for the rejection of doubtful 

 observations,^ or what is equivalent — for the recognition of the disturbances — they 

 following a different law from the general one. The average mean deviation of 

 the readings composing an hourly mean I find = +^ 46', and for 17 values aP' = 4.55; 

 hence readings deviating from the mean more than 1° 38' or 123d are to be recog- 

 nized as disturbances. 



The table of hourly readings contains 23 such values, or one disturbed observa- 

 tion for every 18 ordinary readings. In the five years of hourly observations ending 

 June 30, 1848, at Toronto, the disturbances averaged one in 17 of the whole body. 

 Excluding the above 23 values from the mean, the diurnal inequality freed of the 

 disturbances undergoes no material change, as shown by the following table -. — 



The maximum west deflection is displaced from noon to one o'clock. The general 

 mean changed from 317.3^ to 316.5*, and the range of the mean inequality from 

 1° 06'.8 to 1° 09'.5. Eleven deflections were towards the east and twelve towards 

 the west. The limited number of observations renders it necessary to conclude the 

 foregoing examination of the disturbances. 



Aurora Borealis. — In connection with the disturbances, a short notice of the 

 auroral displays witnessed at the winter quarters will here find an appropriate place. 

 In conformity with the supposed periodicity of this phenomenon, as recognized by 

 Prof. Olmstead, no brilliant and complete auroras have been seen ; with an excep- 

 tion of a very few, they may all be placed in his fourth class, to which the most 

 simple forms of appearances have been referred. The aurora of October 24, 1854, 



* See Vol. III. of the Magnetical and Meteorological Observations at Toronto, Canada, 

 by Major-General E. Sabine. London, 185T. 



' See Gould's Astronomical Journal, Nos. 45 and 83. 



Discussion 



