The use of the slide rule on many classes of work has a desirable psychological effect, in that it calls attention to the accuracy of the data and assists in preventing unnecessary detail work which it is very easy to drift into if any assumptions of great accuracy are permitted to creep in.

The question of significant figures in statistical work and even in ordinary commercial reports is an important one which should have greater attention than it ordinarily receives. Unfortunately the subject can be only briefly touched upon here and the reader would do well to look the matter up in some of the books on statistical theory.*

It sometimes happens that a few blue prints are required from some complex chart made on heavy paper or cardboard. Instead of making a tracing from the drawing by means of tracing cloth and with great expenditure of labor and time, it is sometimes feasible to treat the original drawing with a transparentizing solution so that blue prints can be obtained directly. The transparentizing solution is put on the paper with a brush or sponge and then blue prints are made in the ordinary manner. There are several different makes of the trans-

* A chapter on " Approximation and Accuracy " will be found in " The Elements of Statistical Method," by Willford I. King published by the MacmiUan Company, New York City.

GENERAL METHODS 329

parentizing solution on the market and a supply can be obtained from most shops selling drafting materials.