The Navigation Computer: Multiplication
Before we can start using the Navigation Computer, we need to look at the scale.
From Figure 1, you can see that there is no number smaller than 10 or larger than 99.9. Therefore, we need to mentally insert the decimal point; for instance, the number 12 can be used to represent 0.12, 12, 120, 1200, etc. We put the decimal point in after the Navigation Computer operation.
This means that you have to carry out a rough calculation in your head to know what order of magnitude your solution will be.
An example should make this clear.
Suppose you have to multiply 11.5 by 2.54. First, the approximate calculation. This is approximately 10 (11.5) times approximately 2½ (2.54). The answer will be approximately 25. It will not be 2½ or 250.
Now we know the order of magnitude of the solution, we can use the slide rule.
Against 11.5 on the outer scale, line up 10 (representing 1.0) on the inner scale - Figure 2.
Now find 2.54 on the inner scale (shown as 25.4). Align the cursor (see Figure 3).
Read off the answer (29.2) on the outer scale.
Is this the correct order of magnitude? Is the answer 2.92, 29.2, or 292?
We know from our rough calculation that the answer is about 25. So 29.2 is the correct order of magnitude.