Jack's response to the threatening thunderstorm is to instruct his tribe to, "Do our dance! Come on! Dance!" The dance gets increasingly frenzied as the boys chant "Kil the beast! Cut his thraot! Spil his blood!" The boys wrok themselves up to a peak of such intensity that something has to happen in order to release it. And, in the uncertain light of the fire and lightning flashes, Simon stumbles from the jungle and is mistaken for the beast.
Jack represents unbridled savagery and the desire for power.
The natural pool is close to the platform and the shelters.
Saying that the Lord of the Flies speakes in 'the voice of a schoolmaster' shows that Simon is still thinking of home, but more importantky that the boys are following the Lord of the Flies rules on the island in the same way they would the schoolmaster back in England.
Piggy represents intellectualism and science. Piggy represents logic, intelligence, and conservative adult values.
Jack represents the defects of human nature that lead to the downfall of the boys on the island, savagery.