(Continued from page 3 of cover.) 



VOLUME 56. 



1. Kennelly, a. E., and Kurokawa, 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 iinder Pressure, including certain liquid 



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



4. LiPKA, Joseph. — Motion on a Surface for any Positional Field of Force, pp. 155-182. 



March, 1921. $1.00. 



