10 CARL STØRMER. M.-N. Kl. 
equipotential line through that point. If we call the projection of the 
force K,along M N, as before, K,, we have 
K, = K cos y 
Now since 
2 
- v* 2U 
K = = — , 
09 05 
and 
å vu 2U 
eo ==" — mn Fr 
(a 9 
we obtain 
de Ore 
2 cos wy’ 
whence we have the construction of o shown in fig. 5, when o, and the 
angle yw are given. 
This ‘construction is to be recommended, when there is a suspicion 
that the formula @ = pD does not give exact results, owing to the fact that 
the path is becoming perpendicular to the equipotential curves, or because 
the course of these curves ts not sufficiently regular in the vicinity of the 
point M. By the aid of the already-mentioned cross-rule gelatine-paper, 
on UM He å 
the factor — — is quickly found as follows. 
cos y 
A cross of axes is made on the gelatine-paper, consisting of two 
axes at right angles to one another, one an axis of abscissæ x'x", and 
the other an axis of ordinates vy’. A needle is then put through the 
intersection of the axes and the point M in the path, and the gelatine- 
paper is turned in such a manner that the axis x'x" is tangent to the 
path. Along the normal M N, a portion, M N', is then marked off, of a 
given length a, and N' is marked upon the paper by making a hole with 
a needle through the gelatine. The gelatine is then turned about M until 
