400 
BAUER. 
originator of these lines, which he continually refers to as 
“Lines of equal Dip,” and even terms “Magnetick Paral¬ 
lels,”* consists in the fact that his book contains two small 
maps facing page xxviii, showing the lines of equal dip 
for every quarter of a degree for southern England and 
northwestern France. 
These lines were based upon observations chiefly made 
by Whiston himself for the express purpose of drawing the 
isoclinics to be used in his method of determining geograph¬ 
ical positions. The first map shows the isoclinics resulting 
from dip observations made in 1719, with a needle 12 inches 
long, and embraces a district lying between latitude 48J° 
north and 55|° north and longitude 2°.5 east and 2°.5 west of 
London. The second,f covering the region lying between 
latitude 49° north and 54° north and longitude 2° east and 
3° w^est, shows isoclinics based upon more numerous observa¬ 
tions made with a 47J-inch needle.t 
The values at the various places are also tabulated on 
this second map. The results from the two needles differ, 
on an average, by about 1°.5, the longer needle giving the 
larger value. To counteract the error due to flexure of the 
long needle, Whiston placed a small “Poise of brass circular 
Wire” on the north end of the needle, which required shift¬ 
ing to and fro according to dip. He also says (p. 49) that 
the results with the 12-inch needle were too small, and that 
those with the long, 4-foot needle, agreed with those of the 
small needles (4J and 6 inches), but he nowhere gives a 
comparison. 
Whiston believed that the larger the needle, the better the 
result, if the needle be “ poised ” as stated; but experience 
has not supported this belief, which appears to rest upon a 
false assumption. Dr. Felgentraeger in his article adopts 
* See pp. 53, 55, and 58. On page 53 he defines the magnetic meridians 
as the “ Perpendiculars to the Magnetick Parallels of equal Dip.” On 
page 54 he speaks of “ Magnetick Equator.” 
f Dr. Felgentraeger has given, in his article, a fac-simile reproduction of 
this second map. 
t See Whiston, pp. 91 and 92. 
