William Callaghan & Co. are recorded working from circa 1860 to 1900, at three premises in Bond Street. They were at 23a Bond Street in 1894.
J.L. Gay-Lussac (b.1778-d.1850) made his improvement to the siphon tube barometer in 1816. The short limb of the tube is sealed at the top after the mercury is introduced and a small hole is made an inch or so below the seal, allowing air to get in but not letting mercury escape. The bent part of the tube is contracted to a capillary bore so that when the barometer is inverted capillary attraction keeps the mercury in the long limb. For a full account see Edwin Banfield, Barometers, Stick or Cistern Tube, Baros Books, 1985, p.203.