72 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVI.) 
hence a change near at hand may overpower that at a distance, and a cloud close to 
a station may for the moment do more than the rising sun. These are the irregular 
variations ; and the extent of their influence is well shown by the photographic records 
of Greenwich and Toronto. The volume of Greenwich Observations for 1847 con- 
tains a photographic record of the declination changes, February 18 — 19, 1849. Be- 
tween 6 and 7 o’clock there is a variation of 16' occurring in 18 minutes of time, or at 
the rate nearly of 1' for each minute of time. The course of the mean variation for 
the same date and time is 1'‘95 in two hours, or at the rate of 1 second for each minute 
of time, so that the irregular variation (which may be considered as a local variation 
in respect of the sun’s power for the time) is sixty times that due to the effect of the 
great resultant; moreover it was in the reverse direction, for the temporary variation 
was from east to west, whilst the mean variation was from west to east. 
2946. Another mode of showing how much the action of nearer portions of the atmo- 
sphere overpower and hide the effect of the whole mass, is to draw the line of mean 
variation for the twenty-four hours through such a photographic record as that 
just referred to, and then it will be seen in every part of the course how small the 
mean effect on the needle is, compared to the irregular or comparatively local effect 
for the same moment of time. The magnet with which these observations were 
made, is a bar of steel 2 feet long, 1^ inch broad and a quarter of an inch thick, 
and therefore not obedient to sudden impulses ; it is probable that a short, quick 
magnet would show numerous cases in which the irregular variation would be several 
hundred of times greater than the mean. Still all these irregularities and overpower- 
ing influences of near masses are eliminated by taking the mean of several years’ 
observation, and thus a true result is obtained, to which the hypothesis advanced may 
be applied and so tested. 
2947 . Returning for a short time to the annual variation (2882.), I may observe, 
that it has been a good deal considered in discussing the daily variation. The 
arrangement of the magnetic effects by Colonel Sabine at Hobarton, Toronto, St. 
Helena and elsewhere, into monthly portions, proves exceedingly instructive and im- 
portant, especially for places between and near the tropics. It supplies that kind of 
analysis of the annual variation which is given by the hours for the daily variation. 
Every month, by a comparison of its curve with those of other months, tells its own 
story, at the same time that it links its predecessor and successor together. 
2948. I shall have occasion to trace these monthly means hereafter ; but in the 
meantime refer to the effect of the sun’s annual approach and recession indicated by 
these means, as according with the hypothesis in respect of near and distant actions 
(2945.). Hobarton and Toronto are in opposite hemispheres, so that the sun whilst 
approaching one recedes from the other, and the amount of variations therefore 
changes in opposite directions. Below is the average for each month, derived in the 
case of Hobarton from a mean of seven years, and in that of Toronto from a mean of 
two years. 
