Met. Office Pilot Balloon Slide Rule MK II
The Mark II 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. Mark II rules were also manufactured in Australia by W & G (Melbourne). The W & G rule differs only slightly from the English A. G. Thornton LTD rule. The W & G rule uses serif fonts and has some fiducial marks that the English rule lacks. Exact dates of production are not known but the MK II is known to have started production by 1927 and I have a 1943 example. The printed scales are engraved in the plastic faces. The slide rule has 4 captive cursors (glass windows and brass guides) that ride in slots on the top and bottom edges. Each can be moved independently of one another. Brass knobs at both ends of the slide facilitate its movement. The case is fabric covered wood with felt lining.
This slide rule contains a engraved notation on the rear face "IMPORTANT. NORMALLY NUMBER OF GRATICULE SCALE DIVISIONS PER RADIAN (K) X LENGTH OF TAIL IN FEET (I) = 12X 105. IN OTHER CASES MULTIPLY GRATICULE READINGS BY 1.2/KI BEFORE CALCULATION ON RULE"
The Met Office instructions for use of modified pilot balloon slide rule include directions for calculations using the tail method. These instructions date from 1927. A large image showing the scales of the Mark II slide rule is also posted
Pilot Balloon slide rules in the series were made for the British Meteorological Office by different makers, this one by A. G. Thornton LTD. Alexander George Thornton started originally in partnership with Joseph Halden in Manchester in 1878 and were a well known company making slide rules and drawing instruments.
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