This note reports a sensitive method by which the uniformity of temperature in cry os tats (or other constant-temperature baths) may be determined. The method involves the measurement of differences of saturation pressures of a suitable substance (which need not be critically pure) by means of a very easily constructed differential manometer. Such an instrument is capable of different.iating t emperature effects at the surface and within the bath, and offers some interesting possibilities in studying low-temperature baths.