A simple all glass manometer, which can be used for measuring the pressures from 0.1 mm. to 0.001 mm. has been described. The manometer consists of a thermometer enclosed in a tube. The pressure measurement is done by measuring the time required to cool the thermometer from a given temperature to the other under a given condition. Using a thermometer of long mercury bulb and limiting the operating temperature range so narrow that the Newton’s law of cooling holds, we get the following relation: (Remark: Graphics omitted.) This relation was well confirmed by experiments using the air and the hydrogen.