369 
iron arising Jrom its rotation. 
These observations show very clearly, that when the centre 
of the plate is in the equator, and its plane also coincides with 
the plane of the equator, the deviation due to rotation is always 
nothing, since the small differences to be observed here in 
the revolutions in opposite directions are only such as may 
justly be attributed to slight errors in the adjustments of the 
centre of the needle or of the plane of the plate, which are 
almost unavoidable. With regard to the several deviations 
in the different columns, I should notice, that they are not 
those actually observed, but derived from them by subtract- 
ing the same number from all the deviations observed in two 
corresponding columns, so that they indicate the same dif- 
ference of deviations in the two revolutions as those actually 
observed, and therefore give the same deviation due to rotation. 
The necessity of this reduction arose from the circumstance 
of my having to adjust the compass to the proper height, so 
that its centre might be in the plane of the plate, while it was 
under the influence of the partial magnetism of particular 
points in the plate ; and having done this, when zero of the 
compass was brought to coincide with the point of the needle 
it was not necessarily in the magnetic meridian, since the 
needle was under the influence of this partial magnetism ; 
and as I wished the deviations to be those from the meridian, 
I reduced the observed deviations as I have mentioned. 
Being convinced that the rotation of the plate in the plane 
of the equator caused no deviation of the needle, I proceeded 
to determine the effects produced by its rotation in other 
planes. In the first set of observations which made, the 
centre of the plate was in the meridian, and its p ane perpen- 
dicular to the plane of the meridian and passing through the 
