Slide rule
A slide rule requires a continual estimation of the order of magnitude of the results. On a slide rule 1.5 × 30 (which equals 45) will show the same result as 1,500,000 × 0.03 (which equals 45,000). It is up to the engineer to continually determine the reasonableness of the results, something that can be lost when numbers are carelessly entered into a computer program or a calculator.