3. Accuracy of the slide rule. From the discussion of § 2 it appears that we read four figures of a result on one part of the scale and three figures on the remaining part. Assuming that the error of a reading is one tenth of the smallest interval following the left-hand index of D, we conclude that the error is roughly 1 in 1000 or one tenth of one per cent. The effect of the assumed error in judging a distance is inversely proportional to the length of the rule. Hence we associate with a 10-inch slide rule an error of one tenth of one per cent, with a 20-inch slide rule an error of one twentieth of one per cent or 1 part in 2000, and with the Thacher Cylindrical slide rule an error of a hundredth of one per cent or one part in 10,000. The accuracy obtainable with the 10-inch slide rule is sufficient for many practical purposes; in any case the slide rule result serves as a check.