37S 
iron arising from its rotation. 
in which the rotation will produce no deviation, or there are 
two, or some even number of such positions. 
I have not been able to determine in all cases experimen- 
tally the situation of the plane in which the deviation due to 
rotation vanishes, or whether there may be more than one 
plane in which this takes place; but all the observations 
which I have made, confirm me in the opinion which I formed 
on comparing the preceding results, that when the centre of 
the plate is in the meridian, there is only one plane between 
the tangent plane and the plane passing through the centre 
of the needle in which the deviation due to rotation vanishes, 
and that that plane is parallel to the equator. 
Another conclusion which we may draw from these expe- 
^riments compared with those in Table I. is this, that when 
the centre of the plate is in the meridian, and its plane per- 
pendicular both to the meridian and equator, then, supposing 
the plate always to revolve in the same direction, the devi- 
ation will always be. in one direction, in whatever point of 
the meridian the centre of the plate may be ; for when the 
centre of the plate is in longitude 90°, latitude o. Table I. 
the plane of the plate has this position, and also when in lati- 
tude 90° S. and 90° N. Table IV. and with the same direc- 
tion of rotation, the deviation will be in one direction in these 
two cases. 
As I had already found, that, when the centre of the plate 
was in the secondary to the equator and meridian, and its 
plane a tangent to the sphere, the rotation caused no devi- 
ation of the horizontal needle ; it appeared to me, that there 
ought to be no deviation due to rotation when the plane of the 
plate was in any other plane perpendicular to this secondary.' 
