This slide rule is from the collection of calculating instruments assembled by Assoc. Professor Allan Bromley. His collection provides examples of most forms of calculating devices made from the early 18th to the late 20th centuries. Slide rules constitute a substantial proportion of this collection.

Over the 19th and much of the 20th centuries the slide rule was the primary instrument for calculation used by many people engaged in the trades and in engineering. Although originally invented in the 17th century, and widely used for gauging (or estimating the quantities of certain products such as alcoholic spirits) it took until around 1850 for the slide rule to become generally popular. However for those engaged in estimating (ie, gauging) various quantities of goods in use in the liquor, building and agriculture trades, and for calculating the excise duty to be paid on these goods, numerous kinds of sliding rules with useful scales were developed.

Specialist slide rules of many kinds were developed over the period of their use. In the 20th century many special purpose slide rules were made from various kinds of paper or card, and from light sheet plastics once plastics became a common place in the later part of the century. These types of slide rules were commonly made as promotional devices while retaining some functional value.

The Blackwell circular slide rule was used mainly in car rally racing. It allowed the navigator to calculate how long it would take to travel a certain number of miles at a certain average speed or the distance travelled over a certain time given an average speed. It could also be used convert Miles to Kilometres given that 50 Miles = 80 Kilometres, or to find out the fuel consumption given the fuel used to cover a certain distance.

The Garford Romer is used in navigation to find the actual location of a grid reference on an Ordnance survey map given the grid lines on the map.

Stephen Jones
April 2008

Mfr:
Blackwell's Metallurgical Works Ltd. (Calculator Department)
Thermetal House, Garston, Liverpool.

http://www.wikipatents.com/gb/707740.html

Slide rule, circular, Blackwell Average Speed Calculator, single-sided, two concentric plastic disks (white), the top one having a pointer (printed in red) attached, rotatable. No separate cursor. With labelled plastic sleeve and instruction booklet.

Also included in the package are a Garford Romer and a booklet. The Garford Romer is a square navigational map grid reference finding aid, it comes with instruction leaflet. The booklet titled Rally Navigation by L.N. Needham (1963), has a list on the inside front cover of other items sold by the publisher Rally Equipment Ltd including the Blackwell calculator.

They all housed in a plastic bag.

Made: 1954 - 1970

Model name, manufacturer, and patent number on slide rule, on top disk, in center, printed in black ' Blackwell Calculator / Made in England / British Patent No.707740 ', on back of lower disk, in lower center ' Obtainable from: / Blackwell's Metallurgical Works Ltd. / (Calculator Department) / Thermetal House, Garston, Liverpool. '. White paper label on back of lower disk, in upper center, handwritten in black ' Bromley / 1983-1 '.

Name on plastic sleeve, printed in red ' Blackwell Average Speed Calculator '.

Title and manufacturer on instruction booklet, printed in black ' Blackwell Average Speed Calculator / Mark 1 / Instructions / Blackwell's Metallurgical Works Ltd. / Thermetal House, Garston, Liverpool 19 '

Logo on Garford Romer, in upper left, printed in black ' Garford Romer / Reg.d Design '.

Title on instruction leaflet, printed in black ' Notes / on the use of / The Garford Romer / ( Regd. Design No. 872947) '.

Title and publisher on booklet, printed in black ' Rally Navigation / by / L.N. Needham / Published by / Rally Equipment Ltd., 295 Edgware Road, Colindale, N. W. 9 '. Manufacturer and notes on back cover, in center ' Printed by / J. B. Shears & Sons Ltd. / 191 New King's Road, London - S.W.6. ', in lower right ' Reproduced from the Ordnance Survey Map with the sanctions of the Controller of H.M. Stationery Office Crown Copyright reserved '.

2010/1/468

1954 - 1970

140 mm