Quick dactyls

In a letter dated February 12, 1778, Gilbert White describes an echo near Selborne that returned ten syllables. The reflector was a hop-kiln in Galley Lane, and the one spot where it worked was on the path to Nore Hill. The test line was Tityre, tu patulæ recubans, and the last syllables came back “as audibly and intelligibly … as the first.” When he tried slow spondees instead (Monstrum horrendum, informe, ingens), only four or five returned.

He measured the distance as 258 yards, which is about 75 feet per syllable. Dr. Plot’s rule allowed 120 feet per syllable, so White notes that his echo “falls short of the Doctor’s, as five to eight.”

The arithmetic is worth doing. 258 yards out and back is about 1,550 feet. At roughly 1,125 feet per second (a guess for a mild evening), that’s about 1.4 seconds of delay. An echo can return only what you finish saying before your first syllable comes back, so ten syllables means speaking at about seven syllables a second. Ti-ty-re tu pa-tu-lae re-cu-bans is exactly ten syllables, and quick dactyls are spoken quickly.

Plot’s 120 feet per syllable works out the same way to about 4.7 syllables a second, an ordinary speaking pace. So the gap between them isn’t in the echo. Plot’s rule assumed ordinary speech, and White’s young gentleman was reciting fast. Five to eight is a ratio of speaking rates. The spondees fit too: they take longer to say, so fewer of them fit inside the 1.4 seconds.

White also guessed that at midnight, “when the air is very elastic,” one or two more syllables might come back. Cold air does slow sound, which lengthens the delay, but only by a percent or two, which is a fraction of a syllable. If midnight helped, it was the “dead stillness” that he also mentions.