Spinning a sphere, like a globe.

Hello, I'm pretty new to this so please bear with me, i'll try to be concise as possible.

Essentially I have a game object, at the moment it's a sphere. And when the mouse hovers over it and moves across, it will cause it to spin. Just like holding a globe and pushing your hand across causes it to spin and the inertia continues after the hand has moved away, until the globe has ceased spinning.

So the code I have sourced is from here.

 var rotationSpeed = 10.0;
 var lerpSpeed = 1.0;
 
 private var speed = new Vector3();
 private var avgSpeed = new Vector3();
 private var dragging = false;
 private var targetSpeedX = new Vector3();
 
 function OnMouseDown() 
 {
     dragging = true;
 }
 
 function Update () 
 {
 
     if (Input.GetMouseButton(0) && dragging) {
         speed = new Vector3(-Input.GetAxis ("Mouse X"), Input.GetAxis("Mouse Y"), 0);
         avgSpeed = Vector3.Lerp(avgSpeed,speed,Time.deltaTime * 5);
     } else {
         if (dragging) {
             speed = avgSpeed;
             dragging = false;
         }
         var i = Time.deltaTime * lerpSpeed;
         speed = Vector3.Lerp( speed, Vector3.zero, i);   
     }
 
     transform.Rotate( Camera.main.transform.up * speed.x * rotationSpeed, Space.World );
     transform.Rotate( Camera.main.transform.right * speed.y * rotationSpeed, Space.World );
 
 }

However it rotates on every axis, what would be the best method to have the object only rotate on the Y axis?

Thanks