The Velocity of Sound in Liquid Helium

The velocity of sound in liquid helium under its vapor pressure has been determined by an ultrasonic method over a temperature range from 4.22\ifmmode^\circ\else\textdegree\fi{}K to 1.76\ifmmode^\circ\else\textdegree\fi{}K. From the measurements obtained, the compressibility of the liquid helium has been computed. At temperatures not too close to the $\ensuremath{\lambda}$-point, the values are in fair agreement with those estimated from thermodynamical considerations, but the expected discontinuity at the $\ensuremath{\lambda}$-transition has not been observed.