Carpenter's Slide Rule by Bradburn & Son

In the 17th century, the British author Henry Coggeshall published a pamphlet describing a linear rule especially for calculations relating to timber. As one of the major early exports of the British colonies in North America and later the United States was timber, it is not surprising that a form of Coggeshall’s rule, called the carpenter’s rule, came to be one of the first slide rules used and sold in this country. The carpenter’s rule was made from two wooden one-foot rules that were held together at one end by a metal joint. Unfolded, one side became a simple two-foot measuring rule. The upper part of the other side contained a groove that held a brass slide, with logarithmic scales on the upper and lower edges of both the slide and the adjacent parts of the groove. The outer edges and lower part of this side commonly were marked with other scales of use to carpenters and spar-makers.

This boxwood carpenter’s rule has a brass joint, a brass cap at the end of one arm, and a brass slide. The cap and part of the wood have worn away at the end of the upper arm, which has the slider. The slide has two identical logarithmic scales labeled B and C. Above it is an identical logarithmic scale on the arm; this scale is labeled A. All three of these scales have a double cycle of logarithms. Below the C scale on the arm is a scale labeled D and also GIRT LINE, which is divided logarithmically and runs from 4 to 40 (in other words, this is not the D or principal scale of modern Mannheim slide rules). The girt line can be used in conjunction with other scales to estimate the volume of timber available from a log. The rule is marked: T. BRADBURN & SON MAKERS WARRANTED BEST BOX.

The lower arm contains scales for making scale drawings that are 1/4, 3/8, 1/2, 5/8, 3/4, and 1 inch to the foot. The other side has a scale of 24 inches along one edge, divided to sixteenths of an inch for 6 inches and to eighths of an inch for the rest of the scale. One arm has a scale numbered by twos from 46 to zero and labeled SQUARE LINE M. The other arm has a scale labeled by twos from 36 to zero and marked E. The M and E scales were used while cutting polygonal sections of wood. The outside edge has two scales, each dividing one foot into 100 parts. All of the scales are crudely divided.

According to Gloria Clifton, George Thomas Bradburn made rules in Birmingham, England, from 1841 to 1852. According to W. L.Goodman, Thomas Bradburn was in business making rules in Birmingham from 1839 to 1870. Thomas Bradburn & Son was in business in Birmingham from 1863 to 1876. Hence, this carpenter’s rule would seem to date from the period 1863–1876. This instrument resembles a carpenter’s rule sold in the United States by S. A. Jones & Co. of Hartford, Conn. (2003.0216.01).

References: Gloria Clifton, Directory of British Scientific Instrument Makers, 1550–1851 (London: National Maritime Museum, 1995), 36; W. L. Goodman, British Planemakers from 1700 (Mendham, N.J.: Astragal Press, 1993), 185–186; Bruce Babcock, "A Guided Tour of an 18th-Century Carpenter's Rule," Journal of the Oughtred Society 3, no. 1 (1994): 26–34.

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