he allowed , therefore , half an hour to pass away ; and as at that moment all was silence in the old castle , as nothing was heard but the eternal murmur of the waves -- that immense breaking of the ocean -- with his pure , harmonious , and powerful voice , he began the first couplet of the psalm then in great favor with the puritans : 'thou leavest thy servants , lady , to see if they be strong ; but soon thou dost afford thy hand to lead them on . '