This letter relates to materials suitable for making compasses for aeroplanes. Such compasses would have to be reliable, able to survive the rigours of flight and not wear out. The letter is an example of the more unusual subjects that the Geological Survey was consulted about during the First World War.
Posters relating to the Survey's contribution to the war effort 1914-1918 can be viewed here
Salter began working at the Museum in 1885 and retired in 1925. He moved to Vancouver, Canada where the medal was presented "for long and meritorious service" by the Lieutenant Governor.
This photograph shows shrapnel and other debris from a German bomb which damaged the Geological Museum at Exhibition Road, London at 11:47pm on 10 September 1940. Edward Bailey in his book Geological Survey of Great Britain records that the bomb "broke half the windows" and a later near miss in 1941 "completed out deglazing."