[LI]
MATHEMATICS OF WOODWORK
The woodworker soon discovers that arithmetic is a very practical and necessary subject. He will meet many problems both in drawing and in actual construction which test his ability and call for some knowledge even of elementary geometry. It is important to be able to estimate from his drawing just how much lumber will be needed. He will soon discover through intercourse with dealers in lumber that there are certain standard sizes, and he should make his designs as far as possible conform at least to standard thicknesses.
Common boards are sawed 1 inch thick. When dressed on two sides the thickness is reduced to 7⁄8 . In planning some part of a structure to be 1 inch thick it is better to make the dimension 7⁄8 inch, else it will be necessary to have heavier material planed to 1 inch and the cost will be that of the heavier lumber, plus the expense of planing. In buying 1⁄2-inch dressed lumber, very often inch boards are dressed down to the required thickness, and the purchaser pays for 1-inch wood, in addition to the dressing. The boy is surprised to find that it costs more for 1⁄2-inch than for inch material. Standard lengths are 10, 12, 14, 16, etc., feet. Widths vary, and as wood shrinks only across the grain—with one or two exceptions—this dimension cannot be depended on, as the amount of shrinkage depends somewhat on the age after cutting. Whenever possible, it is wise to go to the lumber yard and select your own material, choosing boards that are free from knots, shakes, etc. Clear lumber—free from knots—costs more, but is worth the difference.
MEASUREMENTS
A measurement is a comparison. We measure the length of a lot by comparing it with the standard of length, the yard or foot. We measure a farm by comparing its area with the standard unit of surface measure, the acre, square rod, or square yard.
In every measurement we must first have an accepted standard unit. The history of units of measurement is a very interesting one, and its difficulty arises from the fact that no two things in nature are the same. One of the ancient units of length was the cubit, supposed to be the length of a man's forearm, from the elbow to the end of the middle finger. This, like other natural units, varied and was therefore unreliable. As civilization progressed it became necessary for the various governments to take up the question of units of measurements and to define just what they should be.
Our own standards are copied from those of Great Britain, and although congress is empowered to prescribe what shall be our units, little has been changed, so that with few exceptions we are still using English measurements.
The almost hopeless confusion and unnecessary complication of figures is shown in the following tables as compared with the metric system:
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