626 
ME. B. STEWAET ON THE NATIJEE OF THE FOECES 
It thus appears that there are twenty-two cases in which the declination is raised or 
lowered along with the horizontal force, and only seven cases of an opposite description. 
Also there are twenty-two cases in which the declination is raised or lowered along with 
the vertical force, and only eleven cases of an opposite description. Finally, there are 
thirty-one cases in which both forces are raised or lowered together, and only two cases 
of an opposite description. 
There is therefore a decided tendency in the cmwes of all the elements to be raised or 
lowered simultaneously ; but this tendency is stronger between the horizontal and vertical- 
force curves than between either of these and the declination. It may at the same time 
be affirmed that, with the exception of the disturbance of August — September 1859, 
there is no very prominent case in which the three elements do not rise or fall together. 
10. Having thus recorded the general appearance of the curves, I shall now give the 
result of the examination of the simultaneous peaks and hollows ; but it will first be 
necessary to state the method in which this has been conducted. 
Each curve has a zero-line, or line of abscissae, along which the times are reckoned ; so 
that if we wish to find the time corresponding to any point in the curve, we have merely 
to measure its abscissa, the commencement of the zero-line (denoting the moment at 
which the instrument was started) being the origin. 
In like mannei', it is equally easy to find the point of the curve corresponding to any 
given time. 
The accuracy of this process depends, however, it mil be seen, on the assumption that 
the time-scale is constant for the different portions of a curve ; and the folloAving is a 
proof that this is strictly true. 
It will be noticed shortly that this system of measurement has brought out a remark- 
able correspondence between the peaks and hollows of the horizontal force and those 
of the vertical force, which may be said to have failed in no one instance. This may be 
received as sufficient evidence, not only of the physical fact brought to light, but also of 
the constancy of the time-scale, in absence of which no such phenomenon could have 
been observed. 
11. The following Table exhibits the results derived from comparing together the 
peaks and hollows of the declination, with those occurring at the same instant of time 
in the horizontal force. 
When peak and peak occur together, or hollow and hollow, this is termed a corre- 
spondence, the reverse a non-correspondence. 
