Thermographs record temperature over time. Most use a small pen to mark the current temperature. A cylindrical drum with paper turns beneath the pen as time passes. The result is a graph of temperature over the course of a day or more.
While they are not navigational aids, thermographs are important scientific instruments for measuring and predicting the weather, which has obvious implications for sea and air travel.
- Older home thermostats measure the temperature in a way very similar to the way the thermograph does. You can read more about how home thermostats at Indiana Public Media.
- Wikipedia's page the on Thermo-hygrograph.