EARLIEST ISOCLINICS AND FORCE OBSERVATIONS. 407 
of the dip agreeing well with the observed values. Secondly, 
if we draw the isodynamic lines with the aid of the observed 
vibrations, we find that they run approximately parallel to 
the isoclinics, just as they should, and thus mutually confirm 
each other.* Thirdly, the vibration time for London (lati¬ 
tude 51°.5 N., longitude 0°.12 W.) is 60J seconds, while that for 
the most northern station, Saltfleet (latitude 53°.4 N., longi¬ 
tude 0°.19 E.), is 66 seconds. Hence for two stations 2° apart 
in latitude and nearly in the same meridian, and through 
which consequently the same isodynamic line would not be 
likely to pass, the difference in the vibration times is5f sec¬ 
onds. Now, as the force varies inversely as the squares of 
the vibration times, the London and Saltfleet observations 
show that the horizontal force is less at Saltfleet than at Lon¬ 
don, as is the fact. Thus it appears that Whiston’s observa¬ 
tions for relative intensity are consistent among themselves 
and with later and improved results. 
Whiston also endeavored to compare the force of gravity 
with that of terrestrial magnetism, but by mistakes in rea¬ 
soning and computing obtained erroneous results. 
In the following table I have given Whiston’s observations 
of dip and intensity. The geographical positions of the 
stations are taken from Dr. Felgentraeger. The stations are 
arranged according to increasing latitude, but I have indi¬ 
cated by numbers the order as given by Whiston, which may 
possibly be the order in which the observations were made. 
The resulting dips with both needles are given wherever 
possible. The seventh column gives the results that I would 
feel inclined to adopt from the foregoing considerations. 
These results are the means of the two sets of observations. 
Where the observation of the long needle only was at hand, 
a correction that varied from —0°.6 to —0°.9, as obtained 
from those stations where observations with both needles 
are given, was applied in the order of time as indicated by 
* On the sketch (page 403) I have roughly drawn the lines of equal rela¬ 
tive horizontal intensity, taking London as unity. The broken lines rep¬ 
resent these relative isodynamics. They were sketched with the aid of 
the last column of figures in the table on page 408. 
