With a resonance method, the velocity of the temperature waves (second sound) in liquid helium II has been determined as a function of temperature from about 1.4°K to the λ-point. The velocity is found to be zero at the λ-point, reaches a maximum of 20.46 m/sec. at 1.7°K and decreases to 19.80 m/sec. at 1.42°K. The accuracy of the measurements is at least +/-0.5 percent. The results are in good agreement with the theoretical predictions of both Tisza and Landau but tend to favor the former's theory over the latter's. The fact that second sound generated in the superfluid is able to register on a microphone diaphragm immersed in the fluid is discussed.