I recently took a gamble on a job lot of plastic slide rules and other oddments on account of a blurry pocket rule in one corner of the photo. Superficially there was little to distinguish it from the generic two-foot four-fold rule made throughout the 19th century and well into the 20th, but something about its outer scales jogged a memory. Although the outside edges looked like standard inches, the inside divisions appeared to be unusually split into four distinct sections. Looking more closely, these mystery scales seemed to have a reddish tinge to them.
Having taken the plunge at £15, I finally tracked down the source of my vague memory: A G Thornton’s 1907 book Mathematical Drawing Instruments and Materials. Under the entry “Pocket Rule” (transcribed in full below), Thornton describes his recently-designed “Civil Engineer’s Pocket Rule” sporting an unusually comprehensive set of scales that “on the centre outside are clearly divided in red”.
Fortunately, upon arrival the blurry rule turned out to be in surprisingly good condition, well used but with no chips or cracks. The outer scales – often rendered illegible by years of pocket buffing – remain very clear, particularly the red fill which is still bright and crisp. Inside, a rather worn signature “A.G. THORNTON, MANCHESTER” (see top right) places the rule prior to the firm’s incorporation at the end of 1906 and, in the absence of later examples, may reflect a relatively short production run.
It would be interesting to find out how much the rule cost compared to the standard models retailed by Thornton and his larger competitors (see for example the single-bevel example by J Rabone & Sons below). While the 21 scales of the Civil Engineer’s rule are equivalent to a very comprehensive set of architectural and surveying rules, Thornton’s almost apologetic justification of its expense over the cheaper “ordinary type” perhaps reveals why the complex, two-colour design never caught on.
On a side note, it occurred to me that as a prominent slide-rule maker Thornton would have been well aware of the developments in coloured scales that began to be seen at this time, perhaps epitomised by Dennert & Pape’s impressive “Yokota” rule of 1908, retailed by John Davis of Derby (below).
Whether he borrowed the idea from this field, or the two emerged independently, is something that would require further research. For now, here are Thornton’s ruminations on the subject:
Pocket Rule.–To the professional man as well as the artisan, mechanic, or workman, the pocket rule is an absolute necessity, and a word here may be given regarding this, especially with reference to the better class, as required by draughtsmen, architects, civil engineers, surveyors, &c., as certainly within the last few years a number of improvements have been made, rendering these much more serviceable for everyday use than previously, and this change is especially noticeable in the information given with some of the important rules of this class. As an instance, it has long been a standing objection that the bulk of the scales engraved on the best rules were seldom or never required, and that the scales most essential were frequently prominent by their absence. A few years ago the writer designed a pocket rule with the object of embracing the really useful and serviceable set of scales and measurements, and from opinions expressed by those who have had these in use for some time, the rule in question seems to have answered the purpose intended. It is made in best boxwood or ivory, two foot, four fold, German silver mounts and outside plate joints, 1 1/8in. wide; the outside is divided into inches and 1/16, 1/8, 1/10, 1/12, and the single bevel pattern has the following scales inside: 1/8, 1/4, 1/2, 3/8, 3/4, 1, 1 1/2, and 3in., as also 20 and 30 divisions to the inch. The same rule with double bevel inside has two more scales, namely, 40 and 50 divisions to the inch, thus forming a complete and useful series with all divisions and sub-divisions clearly and well defined; each of these rules being also provided with engraved protractor head for giving angles. Again, another of this series has been recently introduced under the name of “The Civil Engineer’s Pocket Rule,” this is made in best ivory only, 1 1/4in. wide, two foot four-fold, with double bevel inside, solid German silver mounts outside joints; the outside of the rule is divided into inches and 1/8ths, while on the centre outside are clearly divided in red the following scales: six inches to the mile–chains, and same in feet; 1/500 in., chains, same in feet; 1/2500 in., chains, and same scale in feet; inside bevel edge divided 1/8, 1/4, 1/2, 1in., 3/8, 3/4, 1/16, and 3/16in., as also scales 20, 30, 40, and 50 divisions to the inch.
The above are so arranged that there is no confusion, the scales included embrace all that are usually required for practical use by professional men. The particulars of these rules are given simply because in every respect they are different to what are usually sold, and it is considered prudent to give details as a guide to selection, but it may be noted that many of ordinary type available are suitable for general requirements, and in some cases are preferred, especially as they come in cheaper and suit the purpose required. There is a point that calls for note here, that any first-class maker will design a rule specially for any particular purpose including measurements or scales, and these will be supplied for a little over the usual list price for ordinary style.
