Note 2 — Chapter I

If V is the velocity of the wave in feet per minute, and V′ is the velocity in miles per hour, then

^{V′ × 5280}/₆₀ = V.

But V′ = √2^₁/_⁴ λ , and V = nλ , where λ is the wave-length in feet and n the frequency per minute; from which we have V′ = ¹⁹⁸/_n, or the rule given in the text.