The “Tarrant Hydraulic Slide Rule”: Flow in Pipes and Circular Sewers (c.1960).
Used to calculate flow in pipes, circular sewers and open channels. Duplex plastic rule with metal end braces. Pipes size based on Hazen Williams formula, open channel side based on Manning formula. The rule was supplied by the Paterson Engineering Co Ltd, Windsor House, Kingsway, London. The box contains a small note with the text
“Janus Series Adjustment of Scales. It is possible to align the Scales and adjust the Slide movement of those models in this series, which have metal end plates. To do so loosen the single screws in each end plate, and then line up the slide and the loosened stock, against the fixed part of the stock which is held by two screws in the end plates. After adjustment tighten screws firmly. BLUNDELL RULES LTD“.
For more detail see http://www.sliderules.info/a-to-z/tarrant/tarrant.htm.