_Number_ is a constructor intended to be used w/ the _new_ keyword in front of it (by convention, capitalized functions are constructors).  The last thing that most constructors do is assign the constructed object to _this_.  If used with _new_ magic happens and _this_ refers to the newly constructed object.  If _new_ is not used, _this_ refers to the global object, and the new object is assigned as a global variable (possibly clobbering something that is already there).  I do not like constructors.  They are a brand of magic that is unnecessary and confusing.  I am happy to say that Douglas Crockford (js guru) agrees w/ me on that point.

Maybe parseInt(..) would be a better choice?  Make sure to pass in a radix of 10, otherwise it treats 0 weird.