I wonder whether the idiom "to set the Thames on fire" is currently in use and universally understood. Will it be correctly understood outside the United Kingdom? Would it be correct to say "to set the Thames ablaze" ?
The hyperbole set the Thames on fire, especially in the negative, emerges in the British press around 1800, already acknowledged as a common expression:
It has often been said in derision of a stupid fellow, that he never would set the Thames on fire. — Morning Post (London), 13 Oct. 1802. BNA (paywall)
It makes an appearance in a slang dictionary of 1811:
He will not find out a way to set the Thames on fire ; he will not make any wonderful discoveries, he is no conjuror. — Francis Grose, Hewson Clarke, Lexicon Balatronicum: A Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence, 1811.
The expression could still be used in a positive sense to mean some outrageous, impossible thing:
Lord Thurlow is a devoted admirer of a certain illustrious personage, who abundantly admires himself ; and he has discovered a fact concerning that said personage, of which no one but his lordship ever suspected him to be guilty ; it is no less an exploit than setting the Thames on fire. — William Henry Ireland, Scribbleomania,1815.
It also appears in a bit of newspaper filler in an 1834 Indiana newspaper:
“Molly,” said a lady to her servant, who was not remarkable for her quickness of conception or general industry, “I think you’ll never set the river Thames on fire.” “No, ma’am,” was the reply,” “1 should be sorry to do any thing so bad.” — Rising Sun Times (IN), 19 July 1834.
While an American editor likely added river for those readers who might not know that London is on the Thames, Molly’s I should rather than the American I would suggests British provenace.
Americans, whether in 1834 Indiana or today, would understand the hyperbole but not recognize it as a common expression in their own idiom.
A Google NGram shows considerable popularity in the second half of the 19 c., dropping after 1940, likely because the London blitz made the notion of a river ablaze far less hyperbolic or humorous. The graph also shows some modern currency, but not nearly what it was in the 19 c.
Setting the Thames ablaze rather than on fire is a less common alternate, but in the positive sense, attests to great popularity:
It is unfortunate that so much time at the fag end of the season has been spent over “Esmeralda,” which is no more likely to set the Thames ablaze than when it was produced by Mr. Carl Rosa in English seven years ago. — Truth 28 (1890), 81.
“You may as well attempt to set the Thames ablaze” is a frequent criticism and very often a perfectly just one. — The Lancet 2, 1 (1901), 96.