Rope Caliper Gauge and Wooden Rule incorporating Slide Rule

I have recently been the recipient of a most generous and interesting gift from a great friend. The two items were thought by him to be the property of a deceased relative who was at sea in the 1890s and were passed to me, because of my interest in maritime history. A great deal can be learned by inspection of these ingenious items and I am hopeful that other Society members may be able to contribute more, possibly even from personal use of similar items.

The first item, as will be seen, is a caliper gauge, with a great deal packed into its 4 5/8 inches (11.6cm) by 1 3/4 (4.5 cm) width. The body is boxwood, with end-grain protected by brass, while the head and stem of the caliper ( A in photo at end ) are of solid brass, held captive in a dovetail shaped slot running the length of the boxwood body. Most obviously (at the bottom of the photograph) the gauge incorporates a 4 inch rule divided into eighths. Stamped into the half moon head of the caliper against the lower edge of the stem of the caliper are the letters "DIA", and that edge of the caliper stem is graduated in inches and eighths to a length of 4 inches; below it, in the boxwood body, is cut a further scale, one inch in length, divided into sixteenths, extending the reading of the eighth scale. In a similar position against the upper edge ( B in photo) are stamped the letters "CIR" and the upper edge of the stem is divided into 12 equal numbered sections, each of which is sub-divided into quarters.

The units of the upper scale are neither inches nor centimetres but the relationship of that scale to the lower scale is governed by the value of p. It is clear therefore that the lower scale was designed to measure the diameter of round objects and the upper scale to display a calculation of the circumference of the measured object and, that the units used, in both cases, are inches.

Coincidently, a question on the The International Marine History Information Exchange Group web-site (which can be found as a link from the Society's web-site) revealed the reason why it would be necessary for a working man to know both diameter and circumference, by stating that diameters were used for wire rope, whereas circumference was used for fibre ropes.

The centre of the boxwood body is stamped with the maker's name "COULSELL & SON ( MAKERS LIMEHOUSE LONDON", and an Internet search seemed the most obvious next step. The search revealed an item in an auction house catalogue, described as a "Rope Caliper" by Coulsell & Son, Makers, Salmon Lane, Limehouse, of similar dimensions to the object in hand, but made in ivory; seemingly a more expensive version of the same tool. Otherwise the search revealed little beyond the fact that a firm of the same name was to be found in the Limehouse area in the first quarter of the 19th Century.

Turning the caliper over, a table is stamped, or cut, into the boxwood body, making the instrument both a measuring device and a valuable look-up table of weight, breaking strain and working load. Twelve columns of figures giving circumferences show the corresponding weight per yard of wire rope, hemp rope and chain, and two others give breaking strain and working load, for each circumference measurement in the table.

The second item, at first sight, is a simple 12 inch long folding boxwood rule, opening to 24 inches, but in fact it has proved the more enigmatic of the two items. In some respects it is of rather superior construction; in others this is undermined by the inclusion of ferrous metal fittings.

The rule is marked in inches and eighths only and is numbered on one edge only. In one half there are two steel pins, which, when the rule is closed, locate into holes in steel plates in the opposite half, strengthening the closed ruler for the unusual function mentioned below.

Opening the rule out, there seems something odd about it, but it is not immediately obvious what that is, until the realization dawns that the numbers read right to left (1 at the right-hand end and 24 at the left). The immediate reaction is to turn the rule round, but the numbers are then, of course, upside down. The only explanation which springs immediately to mind is that the rule was made for a left-handed person.

On the opposite edge of the ruler to the inch scale, are a series of what may be scales with lettering "FORE & THWARTS AFT" "LOWER PART OF THE HEAD" "THE HEEL" "MASTS THAT HEAD THEMSELVES". Some parts of the lettering and the scale or markings with them cannot be easily read, even with a magnifying glass.

Turning the closed ruler over, one 12 inch section (the upper half in the photo opposite) comprises a slide rule with a brass slide in the boxwood stock, with the slide again held captive in a dovetail shaped slot. The other 12 inch section appears again to be some kind of look up table perhaps giving the ratio of one set of spars to another, thus:

FORE TOP 15/16 OF M TOP | SPRITS TOP 3/8 of Bowsprit | MIZON YARD 5/6 of M Yard | FORE TOP YD 18/25 OF Fore Yd |

Unfortunately, there is no maker's name on the ruler, but the Science Museum web-site lists a two foot timber slide rule by Coulsell and Son of London dated to the first half of the 19th century, which sounds as if it may be the same instrument.

From the Internet evidence, it appears that the two items could be of earlier date than originally supposed and, at the very least, that Coulsells were making such tools in the early part of the 19th century. It seems that neither item would respond particularly well to prolonged exposure to damp conditions, as the swelling boxwood would tend to restrict the brass stocks which slide in it. The ruler, as has already been mentioned, incorporates steel pins and plates and indeed has steel end plates, instead of brass, and these have suffered some corrosion. The implication appears to be that neither was intended to be used at sea. Are these tools of a rigger or mast maker, or were they intended for use indoors in a sail or rigging loft? What of the weight of rope and chain, per yard rather than by fathom ? does this point to a non-nautical use? Can any Society member provide more information?

By Gary Hicks

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