(Continued from page 3 of cover.) 



VOLUME 56. 



1. Kennelly, a. E., and Kubokawa, K. — Acoustic Impedance and its Measurement. 



pp. 1-42. February, 1921. $1.25. 



2. Bell, Louis. — Ghosts and Oculars, pp. 43-58. February, 1921. $.85. 



3. Bridgman, p. W. — Electrical Resistance under Pressure, including certain liquid 



Metals, pp. 59-154. February, 1921. $1.25. 



