The units of weight most frequently used in the United States for weighing all commodities except precious stones, precious metals, and drugs are the units of the so-called AVOIRDUPOIS system. Avoirdupois units of weight are as follows:

Precious stones and precious metals are usually weighed in the United States by the system of TROY weight, in which there are 12, rather than 16, oz in the pound. Drugs are weighed by APOTHECARIES weight, in which there are also 12 oz in the pound.

The basic unit of the metric system of weight is the GRAM, which contains 15.432 grains. The GRAIN was originally supposed to be equal to the weight of a single grain of wheat, The gram of 15.432 grains is also used in the avoirdupois, troy, and apothecaries system of weights. Multiples and subdivisions of the basic unit of metric weight (the gram) are named according to the usual metric system of nomenclature, as follows:

A METRIC TON equals 1,000 kilograms, which equals 1.1 short tons.

The Engineering Aid is interested in the weight of his instruments and the pull to be applied to the ends of the tape to give correct linear measurements. The common units of weight in surveying are the OUNCE, the POUND, the GRAM, and the KILOGRAM. The following tabulation gives the relationship between these units:

ANGULAR or CIRCULAR MEASURE is used for designating the value of horizontal and vertical angles. For general use in the measure-ment of angles, the circumference of the circle is divided into some even number of equal parts. The unit of angular measure is the angle at the center of the circle subtended by one of the small subdivisions of the circumference. The various units of angular measure are known as UNITS OF ARCS. In practice these units of arcs may be further expressed in decimal or fractional parts.

The Engineering Aid may encounter three systems of angular measure in the use of surveying instruments. They are the sexagesimal, the centisimal or metric, and the mil system.