Elliott Theodolite - Everest Model
The "Everist" theodolite was designed by Sir George Everest in about 1825 when he was home on sick leave from the Great Survey of India.
The full vertical circle was not needed in the survey from mountain top to mountain top. I believe that this type of theodolite was also referred to as an explorers theodolite because it was light weight.
The theodolite is engraved " Elliott Brothers 50 Strand London". The Websters Signature Database shows that Elliott Brothers were at 56 Strand between 1840 - 1858, 5 Charing Cross 1856 - 1858, 30 Strand 1858 - 1860 before moving to 449 Strand.

I have only found one other reference to an Elliott Brothers instrument made at 50 Strand so I would guess that the theodolite is circa 1840 - 1850.