Go to Pharaoh in the morning, ... and you shall stand by the river's bank to meet him; and the rod which was turned to a serpent you shall take in your hand. Exodus 7:15.
Moses and his brother were next directed to meet the king as he visited the river in the morning, and standing upon its bank they were again to repeat their message to him, and as proof that God had indeed sent them, they were to stretch out the rod over the waters in all directions, thus changing them into blood. It was done, and the river ran blood, and all the water in their houses was changed to blood, the fish died, and the water became offensive to the smell. But “the magicians of Egypt did so with their enchantments,” changing in the same way the water drawn from wells. Still the king hardened his heart and refused to yield. For seven days the plague continued, the inhabitants being obliged to dig wells to supply themselves with water.
Another effort at moving the king was now made. The rod was again stretched out over the waters, and frogs came up from the river and spread over the country—into the houses and bedchambers and ovens and kneading troughs. The magicians with their enchantments appeared to bring up similar animals. The general nuisance soon became so intolerable that the king was earnest to have it removed. But although the magicians had succeeded in producing frogs, they could not remove them. When Pharaoh saw this, he was somewhat humbled and desired Moses and Aaron to entreat the Lord for him that the plague might be stayed. They reminded the haughty king of his former boasting and asked where was now the vaunted power of his magicians; then they requested him to appoint a time for their prayers, and at the hour specified the living cause was removed, though the effect remained; for the frogs, perishing, polluted the atmosphere.
The work of the magicians had led Pharaoh to believe that these miracles were performed by magic, but he had abundant evidence that this was not the case when the plague of frogs was removed. The Lord could have caused them to disappear and return to dust in a moment, but He did not do this, lest, after they should be removed, the king and the Egyptians should say that it was the result of magic, like the work of the magicians.... Here the king and all Egypt had evidence which their vain philosophy could not dispose of, that this work was not accomplished by magic, but was a judgment from the God of heaven.—Signs of the Times, March 11, 1880.
From From the Heart - Page 215