.402 
BAUER. 
time according to the well-known method for determining 
dip by means of the vibration time of a dipping needle in 
the plane of the magnetic meridian and the vibration time 
of the same needle swung horizontally. This method ap¬ 
pears to have been first invented by Whiston, and is given 
by him as an approximate control upon the usual method. 
Hence, if F be the total magnetic force, we have the follow¬ 
ing relation : 
F: F cos dip :: 42.6 2 : 22 2 , 
or 
cos dip = 22 2 -f- 42.6 2 = 0.2667; 
whence 
dip = 74° 32'. 
Now, the mean of the results of the one-foot needle, 73f°, 
and of the four-foot needles, 75J°, is 74° 27 / .5. 
A preliminary interpolation formula (Science, vol. xx, No. 
506) established by myself in 1892 on the basis of the in¬ 
clinations observed at London from 1576 to 1888, viz: 
1= 70°.40 — 3°.98 sin j~0°.7 (t — 1850) + 23°.02j, 
where I is the inclination at the time t, gives for 1720.5 
I .== 74°.1. Graham obtained 74° 42' as the result of ob¬ 
servations March 29-May 2, 1723. 
It seems to me better, therefore, to adopt the mean results 
from the two needles, rather than those from the four-foot 
needle alone. This course gives for London in 1720.5 
dip = 74°.46 ± 0°.5. 
From the above we might conclude that the absolute value 
of either of the Whistoiiian isoclinic maps is impaired to 
the extent of about 1°. The relative value, however, re¬ 
mains and is affected either not at all or but slightly by the 
constant instrumental error— i. e., the direction of the iso¬ 
clinics over England in about 1720 may be accepted as given 
by Whiston. This direction is approximately WNW. to 
ESE.; the present direction, roughly, WSW. to ENE. If 
