How to coerce a list object to type 'double'

The code:

a <- structure(list(`X$Days` = c("10", "38", "66", "101", "129", "185", "283", 
                                 "374")), .Names = "X$Days")

Then a is like

$`X$Days`
[1] "10"  "38"  "66"  "101" "129" "185" "283" "374"

I would like to coerce a to an array of numeric values, but coercing functions return me

Error: (list) object cannot be coerced to type 'double'

Thanks,