Six's thermometer
In the bend of the U is a bit of mercury, a metal that is liquid at room temperature. This is pushed around by the thermal expansion and contraction of the alcohol in the first bulb as it responds to the outside temperature. The vacuum in the other bulb allows free movement of the alcohol and mercury. It is the metal indices that measures the temperature; the mercury shows the temperature reading on both scales. This is unlike a normal mercury thermometer, in which the expansion and contraction of mercury itself indicate temperature.