Abstract: An ultrasonic probe for measuring the velocity of sound in a liquid consists of a tube (12), closed at its lower end, and arranged to extend into the liquid, a reflector (20) alongside the tube but spaced away from it, and a source (30) of ultrasonic waves in the tube. The source is arranged so as to cause an ultrasonic beam to propagate through the tube wall (16), to traverse the liquid occupying the space between the tube (12) and the reflector (20), and to be reflected back. The source (30) can be scanned along the length of the tube to ascertain the variation of liquid properties, the position of the liquid surface, and the presence of emulsion layers and precipitates. The wall of the tube through which the ultrasonic beam emerges may be plano-concave in cross-section to enhance the ultrasonic beam intensity in the liquid.