The instrument, similar to X.65, is a differential thermometer consisting of a glass U-tube, whose vertical arms end in two glass bulbs. The tube, which contains a drop of liquid, is fixed to a small wooden board graduated on the front and resting on a small turned base. When one of the bulbs is heated more than the other, the air contained in it expands, moving the drop of liquid along the tube. This type of thermometer (also called thermoscope) was invented by the American Benjamin Thompson, Count Rumford. The instrument is signed "Par Pixii, Neveu et Secc.r de Dumotiez Rue du Jardinet N.2 A Paris."