  This is the third of a series of papers on investigating the
non-gravitational effects in the motion of long-period comets. The influence of
the non-gravitational effects on the original and future orbital elements, and
in particular on reciprocals of semimajor axes is analyzed.The new orbit
determinations were performed in a homogenous way using the basic collection of
50 nearly-parabolic comets of first class orbits. The data consist of 23
hypothetically hyperbolic comets (sample I), 8 Oort spike comets (sample II),
and 19 'old' comets (sample III). The original and future reciprocals of
semimajor axes for each comet were calculated for strictly gravitational case
and the non-gravitational case in a fully consistent way. Clear shift of the
original reciprocals of semimajor axes toward the more elliptical orbits is
demonstrated. The average values of the original and future energy changes due
to the non-gravitational acceleration are derived for each of three samples
independently. Omission of the NG effects significantly affects the position
and width of the Oort spike. The shift of the Oort spike position from the
standard 1/a_ori value of 2-4 x 10^{-5} AU^{-1} to the value greater than 5.5 x
10^{-5} AU^{-1} is discussed.
