English borrowed thermometer from French thermomètre. Its Greek-derived components concern heat and measurement. The name tells you the purpose of the device while leaving its physical design open.
This view explains the components. They are pieces of one construction, not consecutive borrowing stages.
Sources
- Merriam-Webster · thermometer
Formation of the word.
- Merriam-Webster · therm-
Meaning and history of this component.
- Merriam-Webster · -meter
Meaning and history of this component.