NavList:

Frank. You wrote “Do you know if this technique was actually used to any significant extent? It clearly would work in principle (but I really think a sextant for the observations is overkill). Was it a suggestion in a flight manual, more theory than practice? Did they try it experimentally a few times? Or is there documented evidence in a logbook that this method for measuring height was employed with some regularity on airships?”

 I was asked to read a talk on “The Development of Air Navigation" by Terry Hayward at old Warden, because he was tied up elsewhere.  A similar talk of Terry’s was given  Greenwich 2012, this time by David Broughton who was standing in for him.  Terry’s original text as sent to me was:

“There was a constant danger of flying into the sea especially when emerging out of cloud at night. When the R34 Airship completed its first double crossing of the Atlantic in 1919 war ships were pre-positioned not only as a safety measure but also to provide local weather information, in particular pressure settings. When the R34 crew were outside radio range and could not receive accurate settings they devised an ingenious method to assess their height. They first experimented by lowering a barometer on a line to sea level and then hauling it up quickly – this was not successful. Their successful improvisation was to observe the shadow of the ship on the surface of the sea. Using the sextant they measured the angle subtended by the length of the shadow; knowing the length of the ship was 640ft they calculated their height to be 2,100ft – the aneroid barometer indicated 1,200ft an apparent error of some 900ft, fortunately on this occasion on the safe side. When they eventually obtained an accurate pressure setting from a ship some 50-60 miles away they found that the actual error was 1000ft.  Thus proving the accuracy of their improvised method.”

I ditched the bit about the barometer but left in the bit about the use of the sextant.  It’s clearly come from an account of the flight, but whether direct from the log or apocryphal, I know not.  I’ll try and find out more.  A quick look in Hughes "History of Air Navigation" reveals no mention of the R34.