Abstract
The velocity u1 of ordinary sound in liquid helium has been measured in the temperature range from 1.2° K. to 4.2° K Its value extrapolated to 0° K. is compared with the velocity u2 of second sound at 0.1° K. and a slight departure from the Landau relation u2 = u1/√3 is indicated. The behaviour of u1 in the neighbourhood of the λ-point has been studied in detail