Warning Stone
. Anything that gives notice of danger. Bakers in Wiltshire and some other counties used to put a “certain pebble” in their ovens, and when the stone turned white it gave the baker warning that the oven was hot enough for his bakings.
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Entry taken from
Dictionary of Phrase and Fable,
edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.