According to Dictionary.com, a thermometer is used to measure temperature. A thermometer is often a sealed glass tube containing a column of liquid mercury. The mercury expands, contracts, falls or rises in accordance with temperature changes.
The temperature is read where the top of the fluid coincides with the calibrated scale marked on the glass tube or its surrounding frame.
The earliest known thermometer was invented by Galileo in 1597. In 1617, a Latin form of thermometer was called thermoscopium. Jesuit Father Leurechon coined the French term thermometre in 1624, from the Greek thermos, meaning hot, and metron, meaning measure.