 Complete the following phrase:  A major lesson learned in 1914 was that the early British doctrine of positioning field guns in open or semi-open positions made them vulnerable to enemy artillery fire, and subsequently more use would be made of available sheltered and hidden positions for firing. This made the role of the Observing Officer crucial for 18-pounder batteries engaging targets as they could no longer rely on direct line of sight. These officers suffered high casualties. It is instructive to compare the photograph of the 18-pounder gun in the open at the 1914 