  Study of the tilt angles of solar bipolar magnetic regions is important
because the tilt angles have an important role in the solar dynamo. We analysed
the data on tilt angles of sunspot groups measured at the Mt. Wilson
Observatory (MWOB) during the period 1917-1986 and Kodaikanal Observatory (KOB)
during the period 1906-1986. We binned the daily tilt-angle data during each of
the Solar Cycles 15-21 into different 5-deg. latitude intervals and calculated
the mean value of the tilt angles in each latitude interval and the
corresponding standard error. We fitted these binned data to Joy's Law
(increase of the tilt angle with latitude), i.e. the linear relationship
between tilt angle and latitude of an active region. The linear-least-square
fit calculations were done by taking into account the uncertainties in both the
abscissa (latitude) and ordinate (mean tilt angle). The calculations were
carried out by using both the tilt-angle and area weighted tilt-angle data in
the whole sphere, northern hemisphere, and southern hemisphere during the whole
period and during each individual solar cycle. We find a significant difference
(absolute north--south asymmetry) between the slopes of Joy's Law recovered
from northern and southern hemispheres' whole period MWOB data of area-weighted
tilt angles. Only the slope obtained from the southern hemisphere's MWOB data
of a solar cycle is found to be reasonably well anti-correlated to the
amplitude of the solar cycle. In the case of area weighted tilt-angle data, a
good correlation is found between the absolute north--south asymmetry in the
slope of a solar cycle and the amplitude of the solar cycle. The corresponding
best-fit linear equations are found to be statistically significant.
