17. These are wells, or fountains, without water. He shews by these two metaphors, that they had nothing within, though they made a great display. A fountain, by its appearance, draws men to itself, because it promises them water to drink, and for other purposes; as soon as clouds appear, they give hope of immediate rain to irrigate the earth. He then says that they were like fountains, because they excelled in boasting, and displayed some acuteness in their thoughts and elegance in their words; but that yet they were dry and barren within: hence the appearance of a fountain was fallacious.
He says that they were clouds carried by the wind, either without rain, or which burst forth into a calamitous storm. He thereby denotes that they brought nothing useful, and that often they were very hurtful. He afterwards denounces on them the dreadful judgment of God, that fear might restrain the faithful. By naming the mist or the blackness of darkness, he alludes to the clouds which obscure the air; as though he had said, that for the momentary darkness which they now spread, there is prepared for them a much thicker darkness which is to continue for ever.