GR translation: And when the mice run down...

[Michael Bailey]

It's idiomatic and also a reference to a nursery rhyme:

idiom:
"who knows but what" like Phillip Grayson said, essentially means "who
knows whether or not"

the nursery rhyme goes like this:
(http://en.wikipedia.org/wiki/Hickory_Dickory_Dock)
hickory dickory dock
the mouse ran up the clock
the clock struck one   (the clock made whatever sound it makes upon the hour)
and down he come    (that is, the mouse)
hickory dickory dock

so, see, Roger and Jessica are riding out to get a dog for Pointsman's
sinister purposes.
The atmosphere is being set
-- they want to be cozy in bed but instead are riding out
-- Roger is hunched over like Dracula in his Burberry (a very English
looking coat)
--- Jessica wearing a wool coat and the melted snow on it is striking,
psychedelic, beautiful (like Jessica herself is to Roger)
-- it's late at night and therefore the clock of St Felix is "striking
one" (http://www.saintfelix.org.uk/History.html)
--- the theme of London filled with chiming cathedrals brings Orwell's
book _1984_ to mind

if the clock is striking one, then poetic license not only allows, but
insists upon, a mouse that has run up the clock and now will be
running down!

*BUT WAIT, THERE'S MORE* --

a) if the mouse runs up and/or down the clock at a certain time, isn't
that another example of Pavlovian conditioning?

b) and, if the mice "run down for good" -- idiomatically, "run down
for good" means "run down for the last time - indicating a sort of
dramatic finality, the machine standing triumphant over the animal at
last

c) but, also idiomatically, "run down" means to "succumb to entropy"
as a battery when drained is said to have "run down", or a person when
tired is said to "have that run-down feeling" and so the mouse's
attempts, like those of the human characters, cost it time and life
and energy, and leave it in a greater state of disorganization than it
started at

--- so overloaded is this trope with foreboding that the overloading
itself is a source of humor! (at least, it makes me snicker, among all
the other reactions)