  Variation of nuclear shell effects with nucleon numbers are evaluated using
the modified Bethe-Weizsacker mass formula (BWM) and the measured atomic
masses. The shell effects at magic neutron numbers N = 8, 20, 28, 50, 82 and
126 and magic proton numbers Z = 8, 20, 28,50 and 82 are found to vary rapidly
approaching the drip lines. The shell effect due to one magic number increases
when it approaches another magic number. Thus, shell effects are not always
negligible near the drip lines.
