Called whenever the user presses the mouse button on the scrollview
function _mouseDown(event) {

        // Check whether mouse-scrolling is enabled
        if (!this.options.mouseMove) {
            return;
        }

        // Reset any previous mouse-move operation that has not yet been
        // cleared.
        if (this._scroll.mouseMove) {
            this.releaseScrollForce(this._scroll.mouseMove.delta);
        }

        // Calculate start of move operation
        var current = [event.clientX, event.clientY];
        var time = _getEventTimestamp(event);
        this._scroll.mouseMove = {
            delta: 0,
            start: current,
            current: current,
            prev: current,
            time: time,
            prevTime: time
        };

        // Apply scroll force
        this.applyScrollForce(this._scroll.mouseMove.delta);
    }