Historical and Descriptive

The numbers shown in the diagram indicate the fractional parts of the line A B, and if we take our compasses and place one leg on the line A C, at number 5, and the other leg where the diagonal line cuts the line 5, that distance will be found to be just one-half of the distance between A and B. There is a difference of one-tenth of the whole distance between A B, at every point where the diagonal cuts the cross lines. Or in other words, where the diagonal crosses the horizontal lines, the point of juncture is one inch nearer to the line A C, than the next point lower down.

This is the principle on which the diagonal scale is based, and it will be seen that any fraction of a foot or an inch may be so divided by diagonals that the most minute subdivisions may be ob tained.

The measurements, of course, are always taken along the horizontal lines, and measured from the perpendicular to the diagonal.

With a thorough knowledge of the foregoing, it will be easy to understand that the perpendicu lar is not necessarily limited. It may be made twice or four times the length, and divided into twice or four times the number of parts which would render the diagram to make reading of 200ths and 400ths respectively.

If twelfths of an inch or foot are wanted, all that is required is to divide the height into twelve parts instead of ten, draw the diagonal and the twelfths are there.

In using this scale let us examine it in Fig. 1, and we will see that the other divisions are in inches, so to apply the rule we proceed as follows: For instance, we want one inch and forty-six one hundredths, place one leg of the compasses on the one inch mark and the other leg where the diagonal cuts the line at 4, on the sixth division up. This gives the length required.

The foregoing description and explanation ap ply to the diagonal scale, as well as to the scale on a steel square, and generally accompanies a case of drawing instruments.

It may be well to state here that some new squares recently placed on the market, claim special features in the way of rafter tables, etc., but their scope in this direction is limited and cannot contain the general information that may be obtained from the Standard Steel Square as herein described.

Board, Plank and Scantling Measure.—Perhaps, with the single exception of the common inch divisions on the square, no set of figures on the instrument will be found more useful to the active workman than that known as the board rule.

A thorough knowledge of its use may be obtained by ten minutes study, and, when once obtained, is always at hand and ready for use.

The following explanations are deemed suffic iently clear to give the reader a full knowledge of the workings of the rule. If we examine the Fig. A, in the Frontispiece, we will find under the figure 12, on the outer edge of the blade, where the length of the boards, plank, or scantling to be measured, is given, and the answer in feet and inches is found under the inches in width that the board, etc., measures. For example, take a board nine feet long and five inches wide; then under the figure 12, on the second line will be found the figure 9, which is the length of the board; then run along this line to the figure directly under the five inches (the width of the board), and we find three feet nine inches, which is the correct answer in "board measure." If the stuff is two inches thick, the sum is doubled ; if three inches thick, it is trebled, etc. If the stuff is longer than any figures shown on the square, it can be measured by dividing and doubling the result. This rule is calculated, as its name indicates, for board measure, or for surfaces one inch in thickness. It may be ad vantageously used, however, upon timber by multiplying the result of the face measure of one side of a piece by its depth in inches. To illustrate, suppose it be required to measure a piece of timber 25 feet long, 10 by 14 inches in size. For the length we will take 12 and 13 feet. For the width we will take 10 inches, and multiply the result by 14. By the rule a board 12 feet long and 10 inches wide contains 10 feet, and one 13 feet long and 10 inches wide, 10 feet 10 inches. Therefore, a board 25 feet long and 10 inches wide must contain 20 feet and 10 inches. In the timber, above described, however, we have what is equivalent to 14 such boards, and therefore we multiply the result by 14, which gives 291 feet and 8 inches, the board measure.

Fig. 3 shows the method now in use for board measure. This shows the correct contents in feet and inches. It is a portion of the blade of the square, as shown in the Frontispiece.