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2. " On the Influence of the Moon on the magnetic direction at 

 Toronto, St. Helena, and Hobarton." By Colonel Edward Sabine, 

 R.A., V.P. and Treas. Received Nov. 17, 1853. 



Having noticed the inference drawn by M. Kreil from the mag- 

 netic observations at Milan and Prague, that the moon exercises an 

 influence on the magnetic direction at the surface of our globe, cog- 

 nisable by a variation in the declination depending on the moon's 

 hour-angle and completing its period in a lunar day, the author pro- 

 ceeds in this paper to state the results of an examination, analogous 

 to that pursued hj M. Kreil, into the influence of the moon on the 

 magnetic declination at the three stations of Toronto, St. Helena, 

 and Hobarton. 



The observations employed in this investigation consisted of six 

 years of hourly observation at Toronto, five years at St. Helena, and 

 five years at Hobarton, forming, exclusive of observations omitted 

 on account of excessive disturbance, a total of 105,747 observations. 



The processes are related by which, after the separation of the 

 disturbances of largest amount, the observations were treated, 

 for the purpose of eliminating the variations due to solar influence, 

 and of re-arrangement in a form by which the inequality of the moon's 

 action at the diff'erent hours of each lunar day might be brought 

 distinctly into view. The results are shown in tables exhibiting the 

 amount of inequality at each of the three stations corresponding to 

 each of the twenty-four lunar hours. 



It appears from these results that the existence of a lunar diurnal 

 variation in the magnetic declination is shown at each of the three 

 stations of Toronto, St. Helena, and Hobarton, and that it has the 

 same general character at each, viz. that of a double progression in 

 a lunar day, having two easterly maxima nearly at opposite points 

 of the hour-circle, and two westerly maxima also at nearly two op- 

 posite points of the hour-circle. The extreme elongations are not 

 at precisely opposite points of the hour-circle at any of the three 

 stations, nor have the amounts of the two elongations which take 

 place in the same direction always precisely the same value ; but 

 the slight inequalities in these respects are within the limits which 

 might be ascribed to accidental variations, and might therefore dis- 

 appear with longer continued observations. It is otherwise, how- 

 ever, in the author's opinion, with the disparity between the amounts 

 of easterly and westerly extreme elongations which presents itself at 

 each of the three stations. At Hobarton and St. Helena the westerly 

 elongations have the larger values, at Toronto the easterly (the north 

 end of the magnet being referred to in all cases). 



The times at which the extreme elongations in the two directions 

 take place are not the same at the three stations, and are as follows: — 

 At Toronto the easterly extremes take place about the hours of 

 and 12, being the hours of the upper and lower culminations; at 

 St. Helena the westerly extremes about two hours before the culmi- 

 nations ; and at Hobarton about two hours after the culminations. 

 At Toronto the westerly extremes take place about the hours of 6 

 .and 18 ; at St. Helena and Hobarton the easterly extremes respect- 



