Jacob exercises the appropriate humility by not taking credit for the great power given to him. Were he to take credit himself, he would exhibit pride and would thereby lose his ability to command the elements. For the powers of heaven cannot be controlled nor handled only upon the principles of righteousness…when we undertake to cover our sins, or to gratify our pride…the heavens withdraw themselves; the Spirit of the Lord is grieved; and when it is withdrawn, Amen to the priesthood or the authority of that man (DC 121:36-7). Also notice the similarities between the language of Jacob in this verse (the Lord God showeth us our weakness that we may know that it is by his grace) and the oft quoted scripture in Ether, I give unto men weakness that they may be humble; and my grace is sufficient for all men that humble themselves before me (Ether 12:27). In summary, whenever the faithful, mortal man exhibits weakness, the grace of God is sufficient to make up the difference.