An organ pipe s , closed at one end and containing a gas of density ${\rho }_1$  is vibrating in its first harmonic. Another organ pipe $P_2$ , open at both ends and containing a gas of density ${\rho }_2$  is vibrating in its third harmonic. Both the pipes are in resonance with a given tuning fork. If the compressibility of gases is equal in both pipes, the ratio of the lengths of  $P_1$  and  $P_2$ is