Thanks for your continuing comments gentlemen, much appreciated. By good fortune I've traced an example of the instrument to the Barometer World Museum, at Merton near Okehampton.
Your barometer is stationary and the change in pressure being registered is reflecting the change at your location only. Falling pressure sometimes
indicates the approach of a depression or weather front, whilst rising pressure sometimes
indicates the approach of a ridge of high pressure or anticyclone. I use the word 'sometimes' advisedly as there is also a background diurnal change in pressure mixed into the equation - there is a decrease in pressure late in the night and during the afternoon, and an increase during the morning and evening.
That apart, a single pressure reading does not tell anything about the wind, for that you need a network of barometers located over a wide area, with all the readings made at the same time and reduced to a common level - sea level. If the readings are plotted on a chart isobars (lines of equal pressure) can be drawn, and it's the distance apart and direction of the isobars that gives the indication of wind speed and direction. A simple rule is that if you stand with your back to the wind in the northern hemisphere low pressure is always on your left - the reverse applies south of the equator.
However, an aeroplane mobile is not stationary, and if you placed a barometer in an aeroplane it will react to changes in pressure in two senses:
1. It will fall as the aeroplane ascends and rise as it descends - but that tells you nothing about the movement of weather systems.
For example I've attached a trace recorded by a barograph during an ascent by a DH9 over Bircham Newton on 3 May 1919; the aeroplane took off at 1002 GMT and landed at 1137 GMT. Between those times it climbed slowly to 16000 ft directly over the airfield then made a rapid descent. The heights, in feet, are on the left; 5000 ft (the first height) equates to a pressure of 850 millibars, 10000 ft (the second height) = 700 millibars.
2. The barograph will also respond to changes in pressure as it flies across the terrain, but the changes will be very small relative to the large changes resulting from a change in altitude - as shown in the attachment.
Another thing to remember is that aviators had very little meteorological knowledge at this time - it certainly wasn't included in an observer's training in 1915-16 - but that is immaterial. Even today it is impossible to gain any indication of present wind conditions by just looking at a barometer.