Map.addBaseLayers = function () {
    this.defaultCentre = [4, 48.8, 5]; // centre of W Europe
    this.m_mousePrefix = "Mercator ";
    this.map.projection = "EPSG:900913";
    this.map.maxExtent = new OpenLayers.Bounds(-20037508.3427892, -20037508.3427892, 20037508.3427892, 20037508.3427892);
    this.map.addLayer(new OpenLayers.Layer.XYZ(
        "Relief",
        "http://maps-for-free.com/layer/relief/z${z}/row${y}/${z}_${x}-${y}.jpg",
        {
          buffer: 0,
          sphericalMercator: true,
          numZoomLevels: 12,
          attribution: 'SRTM relief maps from <a target="_blank" href="http://maps-for-free.com/">maps-for-free.com</a>'
        }
    ));
}