링크가 안됩니다

아래 코드 진행하면 clang: error: linker command failed with exit code 1 (use -v to see invocation)

이런 메세지가 뜹니다.

## head.h
#pragma once

// global constant
namespace Constants
{
    const double pi(3.141592);
    const double gravity(9.8);
    // ...
}

----

## test.cpp
#include <iostream>
#include "head.h"

using namespace std;
void dosomething2()
{
    cout << "in test" << Constants::gravity << " " << &Constants::gravity << endl;
}

---

## main.cpp
#include <iostream>
#include "head.h"

using namespace std;

void doSomething2();

int main()
{
    cout << "in main" << Constants::gravity << " " << &Constants::gravity << endl;
    doSomething2();
}

원인을 모르겠습니다 ㅠㅠ