Evaluation Context Of Event Handler String

What is the evaluation context of the setTimeout args below? I have a separate Timer instance for each sprite in my program. As coded, "this.Clock" doesn't work.

/*================================================= ===
Timer.js
================================================== ==*/

function Timer(_sprite, _interval) {
var sprite = _sprite;
var interval = _interval;
var isRunning = false;
var timeoutID = false;


this.Clock = function() { // called after setTimout interval
if (isRunning) {
isRunning = sprite.OnClockTick();
if (isRunning)
timeoutID = window.setTimeout("this.Clock()", interval);
else
this.Stop();
}
}


this.Stop = function() {
if (timeoutID) {
window.clearTimeout(timeoutID);
timeoutID = false;
};
isRunning = false;
}


this.SetInterval = function(_interval) {
this.Stop();
interval = _interval;
isRunning = true;
timeoutID = window.setTimeout("this.Clock()",interval);
}


this.Continue = function() {
if (!isRunning) {
isRunning = true;
timeoutID = window.setTimeout("this.Clock()",interval);
}
}
}
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