The phrase to set the Thames on fire, based on the image of an impossible task, means to do something marvellous, to work wonders. It is typically used in negative contexts, of dependable but unintelligent persons, in the ironic sense to work no wonders, never to distinguish oneself.
It is first recorded in The Field cleared of the Noble Stand: or, Animadversions on the Pamphlet so called (London, 1720), a religious tract by “a Sincere Seeker”:
To whom the Deceit is to be trac’d, is a Tale by itself. It begins with an Outcry, which could scarce have been greater or more ridiculous, if the Noble Stand had been reported to have run away with the Monument, or set the Thames on fire.
Jackson’s Oxford Journal (Oxfordshire) of Saturday 11th March 1769 punned on the phrase:
We are informed that there has been upon an Average for this Half Century past, the enormous Quantity of 400 Chaldron of Coals per Annum sunk under London-Bridge, and that this is the principal Reason why Mr. Wilkes is not admitted into the Court of Aldermen; for was he once to take his Seat in that honourable Court, a Man of his incendiary Disposition might take Advantage of that Sad Calamity, and set the Thames on Fire.
On Saturday 14th September 1771, the same newspaper used the variant to set fire to the Thames:
Extract of a Letter from Reading.
The famous Mr. Britain makes new and wonderful Discoveries every Day. The Destruction of the Dock-Yard at Portsmouth was not the only Object our natural Enemies, assisted by the Ministry, had in View; it was at the same Time determined, by the Permission of General C——y to destroy Woolwich and seize the Tower. Sir George C—l—e had engaged to burn the India House, and Alderman H—r—y had 200 Barrels of Gunpowder under the Bank. In a Word, nothing would have been wanting to complete the Ruin of this devoted Kingdom, but to set Fire to the Thames, and that likewise would have been attempted, had not the Vigilance of your patriotick Chief Magistrate, and active Conservator, destroyed all Possibility of that Hellish Scheme’s succeeding.
In A trip to Calais, a comedy first published in London in 1778, the British actor and playwright Samuel Foote (1720-77) added though he lives near the Bridge to the variant to set fire to the Thames; Luke Lapelle says to Lady Kitty Crocodile:
Matt Minnikin, my lady, an honest burgoise [sic], that lives dans the citè [sic], won’t set fire to the Thames, though he lives near the Bridge.
The English poet and satirist John Wolcot (1738-1819) used to burn the Thames in Sir Joseph Banks and The Emperor of Morocco. A Tale, published in London in 1788 under the pseudonym of Peter Pindar:
Though Sir Joseph is not deep-discerning,
And though, as all the world well knows,
A nutshell might with perfect ease enclose
Three quarters of his sense, and all his learning;
Whose modest wisdom, therefore, never aims
To ﬁnd the longitude, or burn the Thames;
Yet, as to things he sets himself about,
With tooth and nail, like Hercules, so stout,
He labours for his wish, no matter what.
The English antiquary and lexicographer Francis Grose (1731-91) gave the following definition in A Classical Dictionary of the Vulgar Tongue (2nd edition – London, 1788):
Thames. He will not find out a way to set the Thames on fire; he will not make any wonderful discoveries, he is no conjurer.
He added the following:
Burner. He is no burner of navigable rivers; i.e. he is no conjurer, or man of extraordinary abilities; or rather, he is but a simple fellow. See Thames.
This addition is interesting because the phrase has also been used of other rivers. For example, in Deutsches Sprichwörter-Lexicon (1873), Karl Friedrich Wilhelm Wander (1803-79) recorded den Rhein anzünden, to set fire to the Rhine, as the type of an impossible task.
In The Autocrat of the Breakfast-Table (1858), the American author Oliver Wendell Holmes (1809-94) wrote:
It is not generally understood that Cicero’s essay was delivered as a lyceum lecture (concio popularis), at the Temple of Mercury. The journals (papyri) of the day (“Tempora Quotidiana,”—“Tribunus Quirinalis,”—“Præco Romanus,” and the rest) gave abstracts of it, one of which I have translated and modernised, as being a substitute for the analysis I intend to make.
IV. Kal. Mart. . . . .
The lecture at the Temple of Mercury, last evening, […] consisted of an imaginary dialogue between Cato and Lælius. […] Certainly, old folks can’t jump,—break the necks of their thigh-bones (femorum cervices) if they do; can’t crack nuts with their teeth; can’t climb a greased pole (malum inunctum scandere non possunt); but they can tell old stories and give you good advice; if they know what you have made up your mind to do when you ask them. All this is well enough, but won’t set the Tiber on fire (Tiberim accendere nequaquam potest).
Notes & Queries (London) of 25th March 1865 published a letter in which a correspondent signing himself P. surmised that, in the phrase, Thames is an alteration of the noun temse, denoting a sieve:
“He’ll never set the Temse on Fire.”—Many years ago, before machinery was introduced into flour mills for the purpose of sifting the flour, it was the custom for the miller to send it home unsifted. The process of sifting was done thus, but principally in Yorkshire. The temse or sieve which was provided with a rim which projected from the bottom of it, was worked over the mouth of the barrel into which the flour or meal was sifted. An active fellow, who worked hard, not unfrequently set the rim of the temse on fire by force of friction against the rim of the flour-barrel; so that in fact this department of domestic employment became a standard by which to test a man’s will or capacity to work hard: and thus of a lazy fellow, or one deficient in strength, it was said, “He will never set the temse on fire.” The long misuse of the word temse for sieve, as well as the superseding of hand labour by machinery in this particular species of work, may possibly have tended to the substitution of sound for sense, in such phrases as “He will never set the Thames on fire,” the Mersey on fire, or any other river.
This far-fetched explanation has unfortunately been repeated with a remarkable conviction, in particular by the English author William Carew Hazlitt (1834-1913) in English proverbs and proverbial phrases collected from the most authentic sources (London, 1869).