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22 RICHARDSON'S BLUEBOOK OF PROJECTION
\y accurate to write 81 as 92, just as 4 can be written 22. The square root of 100 is that number of which 100 is the square — in this case, 10. Nothingelse need be remembered.
(53) Suppose it becomes necessary to take the square root of 175, and the proper process has been forgotten. A glance will show that it must be greater than 10, since 10 x 10 = 100. On the other hand, the square root of 175 must be less than 20, since 20 x 20 = 400. The number desired therefore is larger than 10 but smaller than 20. It is nearer to 10 than to 20, since 175 is nearer to 100 than to 400. A trial attempt may be made with 13. This results in 13 x
13 = 169, which is too small. Trying 14 gives 14 x
14 -T 196. The required 'square root, (that of 175) therefore must be larger than 13 but smaller than 14 — in other words, 13 plus some fraction. One or two more trial multiplications will show that 13.3 x 13.3 gives 176.89, which is near enough for all practical projection room purposes.
(54) The projectionist need never hesitate to use the power formula, I2R = W, because working it out involves figuring a square root. He doesn't have to figure a square root. He can always approximate it with sufficient accuracy by a few trial multiplications.
(55) The sign V means "take the square root of.'' The phrase, "the square root of 175" r.s more conveniently written as V175.
Magnetism
(56) Certain iron ores naturally possess magnetic properties; a piece of such ore is a magnet just as it comes from the mine, needing no treatment, as other forms of iron do, to make it a magnet. Pieces of such ore dangling from a bit of string which left them free to turn were the first magnets by which ships were steered. Refined iron or steel is not normally magnetic, but can be magnetized in several ways, one of