, above, can be used; but as
increases toward its limit, 1, the factor increases toward some unknown limit. Probably it does not exceed 0.4 even for [Fig. 26], and, for want of accurate knowledge on the subject, it may be taken at 0.4 as a rude approximation.
Finally, when the surface slope is less than the natural slope of the earth, then as it decreases from
to zero, the factor should. decrease from the extreme value (say 0.4) to
. Résal takes this factor at