Met. Office Pilot Balloon Slide Rule Mark I
The Mark I Pilot Balloon Slide Rule was made of wood laminated with heavy white plastic with a length of 24.5-in. and width of 2.5 inches. Introduced in 1915 this rule has double slides and three cursers which can pass over each other. The double slides allow for variations in the assumed rate of ascent of pilot balloons. With the adoption of a standard weight nozzle system, ascent rates were standardized and users were advised to lock the slides together as shown in the photo below. The modified pilot balloon slide rule Mark II has a single slide and the addition two additional scales and a 4th curser which was required to use the tail method of altitude estimation.
This Mark 1 Pilot Balloon Slide Rule was produced in 1935 and examples from 1938 have been seen. Pilot balloon instructions dated 1936 only refer to the Mark II slide rule so it appears that these were still being acquired after they were no longer specified by the Met Office.
A more detailed image of the Mark 1 Pilot Balloon Slide Rule is available here.
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