Two rare pilot's wrist mounted 'aero hypsometers' J.H. Steward Limited, London, circa 1915 Each with circular silvered dial inscribed J.H. STEWARD LTD, 406 STRAND, & 547 WEST STRAND, LONDON, AERO HYPSOMETER, COMPENSATED to centre and with blued steel pointer within adjustable outer concentric scale calibrated for altitude 0 to 10,000 feet divided into one-hundred foot intervals and sub annotated 2 to 8 for each thousand feet, in watch-type case with knurled scale adjustment crown at three o'clock and fitted with wire lugs for wrist straps, the first numbered 4842 and with red arrow marker to the inside of the glass, the second numbered 4789 and with domed plexiglass cover, each 5cm (2ins) diameter and with brown leather wrist straps, (2). The firm of J.H. Steward Limited is recorded on the Science Museum website as established in London by James Henry Steward in 1852 as opticians and scientific Instrument makers. J.H Steward became incorporated as J.H. Steward Limited on 1st February, 1913. The company produced a large range of items for military use and advertised in their catalogues that they produced instruments for 'Reconnoitring, Sketching, Night Marching, Signally, Gun Laying'. They were well thought of with their work to such a standard that they were opticians to 'The British & Foreign Governments, the National Rifle Associations of England, India, Canada & the Colonies and the National Artillery Association'. Technically a hypsometer is an instrument that allows altitude to be measured by ascertaining the exact temperature at which water boils. This utilises the principle that the boiling point lowers with lower air pressure. The present instruments however are essentially aneroid altimeters specifically designed for aeronautical use which have been described as 'Aero Hypsometers' by the maker, probably to differentiate them from other altimeters available at the time.The present lot were during the formative years of air flight and were often issued/used to pilots during WWI as well as airship crewmen. Both appear to retain old/original straps which are noticeable long suggesting that the instruments were strapped either around the wrist of a heavily padded flying jacket or even the thigh of a pilot.