EARLIEST ISOCLINICS AND FORCE OBSERVATIONS. 405 
We have already referred to the “vibration time” of a 
horizontal and of a vertical magnet at London. When 
Whiston observed the dip with his great four-foot magnets 
at various places in England in 1720 he also observed the 
time of one horizontal vibration of these magnets at those 
places for the express purpose of determining the difference 
of strength of the magnetic force in different parts of England. 
Apparently the results were not as accordant as he expected, 
for he says almost nothing about his method. On his sec¬ 
ond isoclinic map he records the observation which are repro¬ 
duced in the table below. The only explanation of them is 
the following passage on page 112: “ The Difference of this 
“ Strength of the Magnetick Power, from its Direction , is most 
“ visible in my Second Map, hereto prefix’d: Where I have 
“ all along set down the Seconds wherein my Needle per- 
“ form’d a single horizontal Vibration at about 120 Degrees 
“from the Magnetick Meridian, in most Places: Whose 
“Squares, when Allowance has been made fer the different 
“ Obliquity of the several Directions as to our Horizon, will 
“ give us the different Strength of that Magnetic Power at 
“ those several Places : As does the Angle of Dip give us the 
“ different Direction of the same Power there. Now at the 
“ first Sight, the former there appears to be irregular, and the 
“ latter regular; as is the Case also of our Terrella. v 
Now, our problem is: How did Whiston observe the time 
of one horizontal vibration 120° away from the magnetic 
meridian and yet have acting on the magnet no other force 
than the terrestrial magnetic component? Why did he ob¬ 
serve in a plane the magnetic azimuth of which was 120° ? 
Why did he not simply observe the time of vibration of the 
dipping needle in the magnetic meridian in order to com¬ 
pare the force at various stations, just as he had done at Lon¬ 
don ? These questions are hard to answer, and are submitted 
for the consideration of physicists. 
Before the Whistonian vibrations can be intelligently dis¬ 
cussed it must be settled w T hether Whiston really did what 
I have indicated. Are the vibration times given such as 
53—Bull Phil. Soc., Wash., Vol. 12. 
