630 
ME. B. STEWAET ON THE NATHEE OE THE EOECES 
from an isosceles triangle than that of either of the forces ; we might even suppose the 
the descending limb of the declination peak to become level, or even to ascend in conse- 
quence of the action of this second force. 
The following is a more precise definition of what is requisite before we can refer the 
action to a pure elementary force. 
Let it be supposed that we are comparing together portions of each of the three curves, 
commencing at the same time, then it may be asserted that these portions are due to the 
action of a single disturbing force when (the origin for each being the commencement of 
the portion of the curve under consideration) for equal abscissae the ordinates of the one 
curve always bear a definite proportion to those of the other. 
20. This is illustrated by the following tracings, which exhibit the movements of the 
three curves on August 11, 1860, from 2.34 p.m. to 5.26 p.m. 
HP Beginning 
... End. 
VF 
D 
21. From all this we perceive that, in order to find what are the pure elementary forces 
at work, we must select suitable peaks and hollows, and measure these accurately. 
The following Table exhibits the results obtained by comparing such peaks and hollows 
of the horizontal force with those of the vertical force, no reference being made to the 
declination. 
