The instrument, similar to inv. 383, consists of a glass U-tube whose arms carry two bulbs. The tube, partly filled with liquid, is attached to a wooden board. The latter, also U-shaped, is graduated on the vertical arms and rests on a small turned base. When one of the bulbs is heated more than the other, the air contained in it expands, moving the liquid along the tube. There are some inscriptions in French. This type of differential thermometer (also called a differential thermoscope) was introduced by John Leslie.