406 
BAUER. 
would be obtained in the horizontal plane 120° (probably 
counted from north through west) from the magnetic me¬ 
ridian ? We can answer this in the following manner: The 
time at London of a single horizontal vibration of one of the 
four-foot magnets, at 120° from the magnetic meridian, is 
given by Whiston as 60J seconds. On page 401 of this pres¬ 
ent article we found that this corresponds to 42.6 seconds for 
the freely suspended magnet. On the other hand, one vertical 
vibration of the same or a similar magnet in the plane of the 
magnetic meridian is given as 22 seconds, while the corre¬ 
sponding dip was observed to be 75J°. Hence, from the fol¬ 
lowing relation 
F:Fcos 75° 10': :V : 22 2 , 
we get x — time of one horizontal vibration as deduced 
from dip observation and time of oscillation of dipping 
needle = 43.4 seconds, whereas the reduced horizontal vi¬ 
bration yielded 42.6 seconds. Therefore we may conclude 
that the vibration times given by Whiston on his map really 
apply to a plane 120° from the magnetic meridian, and that 
they represent the strength of no other force than the ter¬ 
restrial magnetic component. 
The next question is: What reliance can be put upon 
these observations for determining the distribution of ter¬ 
restrial magnetic force over England in 1720? Their abso¬ 
lute value we, of course, cannot determine.* 
Each observation is doubtless affected by a large probable 
error, owing to the omission, by Whiston, of numerous pre¬ 
cautions needful in such observations. Nevertheless it can 
be shown that these earliest relative intensity observations 
possess a value that deserves recognition. In the first place, 
we have already (on page 402) applied a rough control in that 
we, with the aid of the observed vibrations, obtained a value 
* On page 22, in connection with observations for determining the law 
of attraction between magnets, Whiston speaks of having made deflec¬ 
tion observations with needles. He does not, however, give the obeserva- 
tions; had he done so, perhaps a rough value of absolute force at London 
in 1720 might have been obtained. 
