that extraordinary leg of Mynheer von Clam

Update:
See the famous first chapter of Moby-Dick for a good verbal parallel in the similar use of "a-going":  

Let the most absent-minded of men be plunged in his deepest reveries—stand that man on his legs, set his feet a-going, and he will infallibly lead you to water... 

Melville's narrator Ishmael writes of feet set a-going; our wandering dragoon captain has a jingle set a-going, punning on different senses of "feet."



For the longest time I never got the allusion to "an extraordinary leg" in the May 1853 installment of Scenes Beyond the Western Border. 

Yet unstained, bright and cheerful, gayly pattering o'er [1857: splashing 'mong] the rocks,—merry river, knowest thou surely where thou rushest in such haste?

Art careless now, in thy morning, of these pleasant green trees' shade?

Well, [1857: Ah!] be happy while thou mayst, round thy mountain parents' feet; smiling thou, and reflecting every hopeful smile of theirs!

Yes, whilst they shelter, dance in sunshine, now thou mayst—

F.—"Hillo! what are you about? Writing in tune with the merry cotton wood leaves? You will have to frankly confess you have invented a new style." 

C. —"Upon my word I was becoming as curious as yourself; a first unfortunate line set the jingle a-going, and I could not stop it; my "feet" got into such a measure, that they were running off with me, —and my discretion, (somewhat like an extraordinary leg of which I once heard a clown sing.) Shall it stand?—to be laughed at one of these days?"

F.—"You are wonderfully given to personification; particularly of rivers..."

(Scenes Beyond the Western Border, May 1853; and
Scenes and Adventures in the Army)

But yesterday I stumbled on the source while reading the London Atlas review of Moby-Dick (First Notice, as transcribed in Contemporary Reviews, edited by Brian Higgins and Hershel Parker).  Complaining about Melville's "extravagance," the London reviewer refers to the comic song of Mynheer von Clam:

Mr. Melville is endowed with a fatal facility for the writing of rhapsodies.  Once embarked on a flourishing topic, he knows not when or how to stop. He flies over the pages as Mynheer Von Clam flew over Holland on his steam leg, perfectly powerless to control the impulse which has run away with him, and leaving the dismayed and confounded reader panting far behind. (Contemporary Reviews)

So there we have it.

The context in Scenes Beyond the Western Border is yet another scene of writing.  The narrating Captain of U. S. Dragoons presents himself in the act of experimental writing, trying out a "new style" (mimicking Disraeli's claim in Alroy) of metrical prose.  In the act of writing, our narrator is interrupted by his imaginary traveling companion and critic, Frank.  Explaining his lapse into poetry, the narrator claims he could not help himself:  the urge to write verse took control and started "running off," like the "extraordinary leg" of a popular comic song. 

Besides the obvious pun on "feet" as both parts of the body and metrical units, the dialogue here alludes parenthetically to the comic song of the steam-powered "Cork Leg."  Melville's critic in the London Atlas charged that in Moby-Dick, Melville's penchant for rhapsody ran amuck, like Mynheer von Clam "on his steam leg." Now, in reply to criticism of his writing style, the Captain admits his poetic urge took control over his will and ran off "like an extraordinary leg of which I once heard a clown sing."

He walk'd thro' squares and pass'd each shop,
Of speed he went at the very top ;
Each step he took with a bound and a hop,
And he found his leg he could not stop.  (English ballads)  

In March 1834 the Baltimore Gazette advertised a performance at the Baltimore Museum that included the "Cork Leg" routine: "Comic song, Mr. Foster, "The cork leg" (founded upon a celebrated Dutch legend)."

In November 1859, the English comedian Sam Cowell was performing three nights a week at the French Theater in New York,  The NY Herald advertisement promised that, among other routines, Cowell would 

"illustrate the disastrous results obtained by the application of modern machinery to an offending member of the civic fraternity as developed by the adventures of Mynheer Von Clam, while aided by a CORK LEG."

As a child, Cowell had performed in Boston and Philadelphia.  In 1839 Cowell was doing shows in New Orleans.

For the entire ballad of the Cork Leg, check out the 1844 Quaver; or, Songster's Pocket Companion and the online collection of Music Hall Lyrics.

Samuel Houghton Cowell (1820-1864)