horizontal needle when under the influence of magnets. 37 1 
the needle deviated towards the west , the deviation of the 
westerly point of equilibrium from the increase of intensity 
would be minus , and from the change of direction , plus ; so 
that its character, whether plus or minus , depending upon the 
one cause or the other producing the greater effect, would in 
some cases be somewhat ambiguous. With regard to the 
easterly point of equilibrium, under the same changes of in- 
tensity and direction, its deviation from both causes would be 
plus ; and if the intensity and westerly deviation of the force 
increased during the same period, we should expect little or 
no ambiguity in the character of the deviation of the easterly 
point of equilibrium. With regard to the southerly point of 
equilibrium, when that point is towards the east, the increase 
of intensity would cause its deviation to be plus, and the 
change of direction, minus ; so that here again we might expect 
some ambiguity ; but when the southerly point is towards the 
west, these two causes acting at the same time, would both 
tend to make its deviation minus. From this it would appear, 
that at particular points of equilibrium, to the west of north 
and to the east of south, if the intensity of the terrestrial force 
increased with the deviation of its direction westward, the two 
causes might so counteract each other’s effects that little or 
no deviation of the needle at these points would be apparent ; 
but the precise situation of such points would depend in a 
great measure on the nature of the arrangement of the mag- 
nets by which the needle was held in equilibrio at them. 
To proceed with the observations on the daily changes in 
the several points of equilibrium, I have stated that, during 
the time in which I made observations, in-doors, on the needle 
No. 1 , I had likewise observed, in the open air, the deviations 
