RayCast Shooting not working

Hi, I am new to unity this year, and have been having problems with the raycast shooting script. When I play my game, it spawns the effect, but it only spawns in the middle of map. I have had another game without terrain that the raycast worked perfectly. Is terrain the problem? ![alt text][1] Here is my script:

 #pragma strict
 
 var Effect : Transform;
 var TheDammage = 100;
 
 function Update () {
     
     var hit : RaycastHit;
     var ray : Ray = Camera.main.ScreenPointToRay(Vector3(Screen.width*0.5, Screen.height*0.5, 0));
     
     if (Input.GetMouseButtonDown(0))
     {
         if (Physics.Raycast (ray, hit, 100))
         {
             var particleClone = Instantiate(Effect, hit.point, Quaternion.LookRotation(hit.normal));
             Destroy(particleClone.gameObject, 2);
             hit.transform.SendMessage("ApplyDammage", TheDammage, SendMessageOptions.DontRequireReceiver);
         }
     }
     
 }
 
 
   [1]: /storage/temp/35514-raycast+shooting+not+working.png