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 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. 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. The cursors are missing from this item. Brass knobs at both ends of the slide facilitate its movement. The case is fabric covered wood with felt lining. No known history for this item as located in a property after purchase. The Pilot Balloon slide rule is used to convert the observed azimuth and elevation readings into wind velocity and direction records. The use of slide rules was abandoned with the wide spread adoption of computers and programs to do the calculations, as well as the decline in the use of optical pilot balloon theodolites.Made in Australia by White & Gillespie (Melb) Pty Ltd. Established in 1910, the firm was involved mainly in printing plate manufacture, but during the Second World War it manufactured a variety of navigational aids and range finding devices for the military. About 250,000 instruments were made during that period. After the war the company produced, among other items, drawing instruments and slide rules. The case is fabric covered wood with felt fabric lining containing a long black laminate ruler with numerous white markings and numbers. Top of the ruler is in three sections with the centre section movable by small gold metal knobs at either end. 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" Top of Slide Rule - RAAF Ident No G268/480 Serial WG/22pilot balloon slide rule, raaf

Pilot Balloon Slide Rule (case only).equipment, ww2, raaf