A thermoscope is a device that shows changes in temperature. A typical design is a tube in which a liquid rises and falls as the temperature changes.
Galileo Galilei discovered the principle on which the device is based. He made the first known thermoscope around 1592. Giuseppe Biancani published the first clear diagram of a thermoscope in 1617.
Although the device shows changes in temperature, the concept of temperature as a measurable quantity was not developed until later. Adding a temperature scale to a thermoscope makes a thermometer. This was done in 1701 by Ole Christensen Rømer with the Rømer scale, though a reproducible scale had been developed by Robert Fludd in 1638, and the earliest scale was possibly by Francesco Sagredo or Santorio Santorio in about 1611 to 1613.
See also 
- The Galileo Project, "The Thermometer"
- Benedict, Robert P., 1984. Chapter 1, "Early attempts to measure degrees of heat", in Fundamentals of Temperature, Pressure and Flow Measurement, 3rd ed, Wiley ISBN 0-471-89383-8.