Six's thermometer
If the temperature rises, the maximum scale marker will be pushed. If it falls, the moving mercury will push the minimum scale marker. As the temperature varies, the markers will remain in their positions unless the temperature becomes higher (for maximum) or lower (for minimum) than already recorded, in which case the relevant marker is pushed further. For this reason, the markers  record the furthest point reached by the mercury in each arm of the tube. This corresponds to the highest and lowest temperatures since the last reset. Typically the thermometer is reset every day to measure temperature changes.