The Collaborative International Dictionary of English v.0.48:

Thermoscope \Ther"mo*scope\, n. [Thermo- + -scope.] (Physics)
   An instrument for indicating changes of temperature without
   indicating the degree of heat by which it is affected;
   especially, an instrument contrived by Count Rumford which,
   as modified by Professor Leslie, was afterward called the
   differential thermometer.
   [1913 Webster]