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The The Troughton & Simms Pilot Balloon theodolite is from 1921. it is very similar the Watts Mark B theodolite which along with the Cary Porter Model D were in common use by the Metrological Office of the United Kingdom prior to the Introduction of the Met. Office Pattern Theodolite in 1937. While the Cary is an open frame type with 10 part verniers, the Troughton and Simms and the Watts Mark B have easier to read Micrometer Drums. Further like the Watts the Troughton & Simms worm drives both engage and disengage with the single operation of a linked pair of levers, further speeding up operations. This instrument appears not have have provision for lighting.