Fixing the Decimal Point
(See page 1292, August, 1924, PROCEEDINGS)
CAPTAIN ELLIOT SNOW (CC), U. S. NAVY.—During my day at the U. S. Naval Academy—some forty years ago—we were not told of the existence of a slide rule, nor were we given any instruction in methods of quickly approximating the value of computations, even of the simple kind used in illustrating Lieutenant Commander Green's article. We were, however, taught logarithms after a fashion.
Lieutenant Commander Green has performed a lasting and genuine service to a great many of the younger readers of the Institute proceedings, by telling in such simple language how to fix the decimal point in making calculations with a slide rule. The reason for the rule forms, too, a very necessary part of the article.
He has put his finger on the great stumbling block in the use of a slide rule—not knowing how to fix the decimal point—and having pointed this out it is to be hoped that hereafter, teachers at the Naval Academy of the subject of logarithms will include in their instruction, not only how to use a slide rule but in doing so will also explain clearly how to fix the decimal point.
In the rule printed in italics, at the head of page 1294, the author uses the expression "going right across the rule's mid-point." And in the description of the problem which immediately follows on the same page, he uses similar expressions; for example, "so when we divide by 20 we are moving from right to left."
I was puzzled in reading the article, because a slide rule has no middle point, and because I could not decide what it is that "crosses the midpoint [toward the] right." Obscure expressions, such as these, are often found in textbooks .and make students waste much time in trying to understand them. Fortunately, the article closes with a bit of very sound advice, the sum and substance of which is, "Try it without thinking any further of his explanation or my criticism."
The fact that probably over ninety per cent of the engineering calculations that one meets in every-day practice could be made quickly and with sufficient accuracy, using a To-inch slide rule, is sufficient in itself to justify imposing the requirement on midshipmen that slide rules be customarily used when solving problems. This practice, if consistently followed, and if coupled with a short, lucidly explained course in "Precision of Measurements," will go a long way toward saving a great deal of valuable time.