converting derived class pointer to private base class pointer

Two days back, I have posted a question to comp.lang.c++ with the
subject line "derived class and virtual function".
In one of the replies to that question Francesco S. Carta had given
the following program:

#include <iostream>

using namespace std;

class Base {
     public:
     Base(int data = 0) : data(data) {};
     Base(const Base& base) : data(base.data) {};
     virtual Base* Clone() const {
         cout << "cloning Base" << endl;
         return new Base(*this);
     }
     int Data() const {
         return data;
     }
     private:
     int data;

};

class Derived : private Base {
     public:
     Derived(int data = 0) : Base(data) {};
     Derived(const Derived& derived) : Base(derived) {};
     Base* Clone() const {
         cout << "cloning Derived" << endl;
         return new Derived(*this);
     }

};

int main() {

// fails, of course
        Base* pb = new Derived(42);

     Derived d(42);

     // calls Derived::Clone()
     Base* pb2 = d.Clone();

     // calls Derived::Clone()
     Base* pb3 = pb2->Clone();

     // prints 42
     cout << pb3->Data() << endl;

}

When I compiled this program with g++3.4.3 as
g++ -std=c++98 -pedantic -Wall -Wextra a.cpp

I get the following error:

a.cpp: In function `int main()':
a.cpp:35: error: `Base' is an inaccessible base of `Derived'
a.cpp:35: warning: unused variable 'pb'

This error corresponds to the line:
        Base* pb = new Derived(42);
This is because since Base class is private, I am trying to convert
'Derived*' to 'Base*'. However, in the Derived class Clone() function,
I am returning 'new Derived(*this)' as 'Base*'.
ie I am converting 'Derived*' to 'Base*'. How is this accepted in this
function?
     Base* Clone() const {
         cout << "cloning Derived" << endl;
         return new Derived(*this);
     }

Kindly clarify.

Thanks
V.Subramanian