In work of this kind, it is sometimes convenient to use one of the small-size pocket adding machines of which there are several different makes now on the market. Though these machines are not at all in the same class as the large key-operated machines, they are of assistance in taking off occasional items because they overcome the necessity for putting

GRAPHIC METHODS

dov/n the items on a piece of paper for addition later by mental effort. With the portable adding machine the data are taken from the original work directly and added automatically as the work proceeds.

Engineers do the greater portion of all of their computing work by means of the slide rule. By others than engineers, however, the slide rule is very little used. In the preparation of data for curves and charts for corporation work, or for any investigations where numerous percentages are necessary, the slide rule is almost indispensable. A 10-inch slide rule as shown in Fig. 232 is sufficiently accurate for most work, and, as it costs only a few dollars, it should be a part of the equipment of everyone who is doing even the simplest form of statistical work, or who is periodically plotting curves involving ratios or percentages. By using a slide rule the percentage ratios of numbers can be determined almost instantly and with no mental effort. It can be confidently predicted that anyone who has miich multiplication or division to do in relation to curve-plotting and chartmaking will find the slide rule of such a great assistance that the rule would not be parted with under any circumstances if a new one could not be obtained.