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XIX. The Bakerian Lecture. — On the Influence of the Moon on the Magnetic 
Declination at Toronto, St. Helena, and Hoharton. By Colonel Edward Sabine, 
R.A., Treas. and V.P. 
Received and Read Novembei’ 17, 1853. 
The success which attended the endeavour to detect the influence of the moon on 
the pressure of the atmosphere, by a suitable arrangement of the hourly barometrical 
observations at St. Helena*, naturally suggested the idea that the influence of the 
moon on the direction of the magnetic needle, supposing such an influence to exist, 
might be manifested by an analogous arrangement of the hourly magnetical observa- 
tions at that station ; inasmuch as the magnetical disturbances due to other causes, 
and liable to mask so small an effect as that which might be anticipated from the 
moon, were, like those of the barometer, of inconsiderable amount at St. Helena 
when compared with those at many other stations. 
An examination of observations of the Declination made at Milan, whilst M. Kreil 
was Director of that observatory, led him, in a memoir read in February 1841 to the 
Royal Bohemian Society of Sciences, to announce his belief that the moon does actu- 
ally exercise an influence on the magnetic direction at the surface of our globe, 
cognisable by a variation in the Declination depending on the moon’s hour-angle, and 
completing its period in a lunar day. M. Kreil has since confirmed the discovery 
thus announced by investigations based on a more extensive series of similar obser- 
vations made under his direction at Prague, and discussed,— 1st, in the ‘ Magnetische 
und Meteorologische Beobachtungen zu Prag’ for 1841 ; and 2nd, in a memoir pre- 
sented to the Imperial Academy of Sciences at Vienna in June 1850, and published 
in the Transactions of that Academy in 1852. 
Meanwhile, in the course of the discussion of the results derivable from the Obser- 
vations at the British Colonial Observatories, I had selected for a primary examina- 
tion of this subject the series of hourly observations of the Declination at St. Helena, 
extending from September 1842 to August 1847 inclusive; and having determined 
on the process through which the observations should be passed, the work of reduc- 
tion was commenced, and tliough occasionally interrupted by more pressing duties, 
was resumed from time to time, and was at length completed for the five years in the 
early part of the summer of 1852. The result was conclusive, in so far as a variation 
depending upon the moon’s hour-angle was systematically and consistently mani- 
fested ; but this variation differed so considerably in many important particulars, as 
* Philosophical Transactions, 1847, Art. V. 
