Balloon (A pilot).
Metaphorically, a feeler, sent to ascertain public opinion.
“The pilot balloon sent from … has shown [the sender] the direction of the wind, and he now trims his sails accordingly.”—Newspaper paragraph, January, 1886.
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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.