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XV, On Periodical Laws discoverable in the Mean Effects of the larger 
Magnetic Disturbances. — No. III. 
By Colonel Edward Sabine, R.A., D.C.L., Treas. and V.P.R.S. 
Received February 6, — Read February 14, 1856. 
Having at length completed the analysis of the larger disturbances of the hori- 
zontal and vertical magnetic forces at Toronto during five years of hourly observa- 
tion, with a view to the development of the periodical laws which regulate the occur- 
rence of the occasional disturbances of those elements, and of their theoretical equiva- 
lents, the Inclination and Total Force, I now propose to lay before the Royal Society a 
condensed view of the mode in which the investigation has been made, and of its 
results. 
The hourly observations of the Bifilar and Vertical Force Magnetometers during 
the five years terminating June 30, 1848, were received at Woolwich, from Toronto, 
precisely in the state in which they are printed in the second and third volumes of 
the ‘ Observations at the Toronto Observatory’ ; namely, the readings, uncorrected for 
temperature, at every hour of Gottingen time, arranged in Monthly tables, accom- 
panied by corresponding tables of the temperature of the magnets, shown by thermo- 
meters of which the balls were enclosed in the same case with the magnets, and 
which were read contemporaneously with the Bifilar and Vertical Force scales. The 
Monthly tables of the scale-readings and of the temperatures were summed before 
their transmission to Woolwich, both in vertical and horizontal columns, and means 
were taken of all the days in the month at the different hours, and of all the hours of 
the day on the different days, forming hourly means” and “ daily means.” In this 
state the observations were received at Woolwich and subsequently printed ; they 
were, in fact, printed from the original manuscripts. 
The first step taken at the office at Woolwich was to rewrite the whole of the 
observations of the five years in scale-divisions, corresponding to the respective 
readings, but reduced to a uniform temperature of 55°, which was taken as a conve- 
nient approximate mean temperature : for this purpose each of the observations had to 
receive a correction, proportioned to the difference between the recorded contempo- 
raneous reading of the thermometer and the standard temperature of 55°, and com- 
puted by a coefficient representing the change in the scale-reading produced by an 
alteration of 1° of the thermometer. The coefficient was obtained directly from the 
observations themselves, by ascertaining the factor which would best satisfy the differ- 
3 B 
MDCCCLVI. 
