This is a fine antique rare pocket Barometer / 15.000 Ft Altimeter compensated 6683  by Louis P Casella maker to the Admiralty and ordnance London c1865 . The Barometer has a rotating bezel with a silvered dial and a gilded case with a blue steel pointer.The diameter is 50mm - 2"  it has been plastic bag pressure tested and works perfectly the Barometer and the case are in excellent condition you will not find a better example.