  We have mapped the \hcn emission from two spiral galaxies, NGC2903 and
NGC3504 to study the gas properties in the bars. The HCN(1-0) emission is
detected in the center and along the bar of NGC2903. The line ratio
HCN(1-0)/CO(1-0) ranges from 0.07 to 0.12 with the lowest value in the center.
The enhancement of HCN(1-0) emission along the bar indicates a higher fraction
of dense molecular gas in the bar than at the center. The mass of dense
molecular gas in the center (2.2x 10^7 Msun) is about 6 times lower than that
in the bar (1.2x 10^8 Msun). The total star formation rate (SFR) is estimated
to be 1.4 Msun/yr, where the SFR at the center is 1.9 times higher than that in
the bar. The time scale of consumption of the dense molecular gas in the center
is about 3x 10^7 yr which is much shorter than that in the bar of about 2 to 10
x 10^8 yr. The dynamical time scale of inflow of the gas from the bar to the
center is shorter than the consumption time scale in the bar, which suggests
that the star formation (SF) activity at the center is not deprived of fuel. In
the bar, the fraction of dense molecular gas mass relative to the total
molecular gas mass is twice as high along the leading edge than along the
central axis of the bar. The \hcn emission has a large velocity dispersion in
the bar, which can be attributed partially to the streaming motions indicative
of shocks along the bar. In NGC3504, the HCN(2-0) emission is detected only at
the center. The fraction of dense molecular gas mass in the center is about
15%. Comparison of the SFR with the predictions from numerical simulations
suggest that NGC2903 harbors a young type B bar with a strong inflow of gas
toward the center whereas NGC3504 has an older bar and has already passed the
phase of inflow of gas toward the center.
