Sing, O heavens (comp. Isaiah 44:23). Heaven and earth are called upon to rejoice and "break forth into singing"
O mountains The majesty of mountains seems to have deeply impressed Isaiah. Throughout his writings they are continually introduced as the grandest of the works of God (comp. Isaiah 2:2, Isaiah 2:14; Isaiah 5:25; Isaiah 13:4; Isaiah 14:25; Isaiah 22:5; Isaiah 30:25; Isaiah 34:3; Isaiah 37:24; Isaiah 40:4, Isaiah 40:9, Isaiah 40:12; Isaiah 41:15; Isaiah 42:11, Isaiah 42:15, etc.). He expects them to be especially ready to sympathize with man. Such a feeling would be natural to one accustomed to the hill-country of Palestine and the lofty heights of Hermon and Lebanon, but could scarcely have been developed in an exile of the time of Cyrus, born and brought up in the dead level of Babylonia. Hath comforted … will have mercy. Both verbs designate the same action, which is really future, but in God's counsels is already accomplished. The perfect is thus, once more, that of prophetic certitude.