316 Mr. Christie's Iheory the 
of the heated point were liable to errors arising from the 
first circumstance, as well as from adjustment; also, that the 
needle not always coming to rest in the same time, it was not 
possible to make the successive observations at precisely the 
same intervals throughout. Such care was however taken 
to guard against this last source of error, that each set was 
made in very nearly the same time: the whole set, when the 
point o was heated, occupied i8 minutes ; that when 180 was 
heated, 18 minutes; that when 90 was heated, again 18 minutes; 
and that when 270 was heated, 19 minutes. The observations 
then clearly show that the deviations of the needle, corres¬ 
ponding to the several positions of the heated point, did not 
arise from a peculiarity in the arrangement of the particles, 
or in the contact of the two metals, in particular parts of the 
plate ; but that to whatever polarity in the plate, or to 
whatever electric currents these deviations were due, these 
poles were symmetrically situated, or these currents were 
uniformly excited, to whatever point in its circumference 
heat was applied: and that, consequently, whatever might be 
the results obtained by applying heat in all cases to the same 
point in the circumference, they would not dilfer sensibly 
from the mean of the results obtained from corresponding 
observations, if heat were applied to several points succes¬ 
sively, as in the foregoing instance, in each particular case. 
To have adopted the latter method in every particular posi¬ 
tion of the compass and the plate, in which I proposed to 
investigate the effects, would indeed have been almost an 
endless task: however, to leave no doubts respecting the 
results which I might obtain, whenever the deviations were 
of a nature that might possibly arise from a want of uni- 
