A simple question about bootstrap in R

Hi,

I am using bootstrap package in R library but got confused about the
syntax.

Here is the short sample code:

library(boot)
ratio <- function(d, w)
sum(d$x * w)/sum(d$u * w)
boot(city, ratio, R=999, stype="w")

My question is how the arguments are passed to the function "ratio". I
can vaguely see that "city" is passed as "d" but where is "w"?

Thanks for your help!

.