make the tops of every other row of keys concave, so that one can instantly tell by the feeling which row he is striking. All the keys standing for odd numbers have concave tops, and all standing for even numbers have flat tops. It requires considerable practice to acquire sufficient skill to enable one to use the Comptometer rapidly and accurately on addition. It is not to know how, but it is to acquire skill of the fingers, which requires practice. In this respect it is precisely like the typewriter.
Twenty minutes' use of the Comptometer each day for sixty days will develop more speed than sixty hours' practice all in a few days. It requires more practice to perform addition rapidly than it does to do any other class of examples. Doing actual work is the only practice that is good for anything. But as addition constitutes the bulk of mathematical work, the Comptometer is used for addition more than for anything else. One should not try to do all their practicing of addition in one day, but use the machine twenty minutes each day for several weeks, when they will find that they can easily add with the Comptometer more rapidly and correctly than any two men can mentally. Do not imagine that you can estimate what you will be able to do after more practice, when you have only practiced ten days, for at that time you will just be beginning to learn how to use the machine, although you may think yourself quite an operator.
To perform addition, it is necessary merely to strike each number on the machine and the total will be indicated on the register.
It is not necessary to strike the figures in regular order, but for convenience and speed it is better to strike the small figures in each number before striking the larger ones. Thus: In striking the number 263, first, with the first finger, strike 2 in the third column; next, with the third finger, strike 3 in the first column; with the second finger, strike 6 in the second column. The movement of the hand back and forth over the key-board several times is thus avoided, and the key next to be struck is always exposed to view.
Strike each key firm and distinct, raising the finger clear up before putting another finger on another key.
