Why did Mrs. Putnam ask Ruth to get Tituba to take the girls into the forest on the evening before Betty and Ruth became sick/unconscious?

 Mrs. Putnam has lost a number of babies-- (nowadays it might be stillbirth or SIDS) She hopes that Ruth can have Tituba conjure the spirits of the babies.

Explanation: Tituba is a servant to Betty's father. She comes from Barbados where they have different dances and rituals. The girls seem to join in for fun, but there seems to be a spiritual aspect to her practices that the Puritans regard as satanic.  

the bottom of the ocean

where is the writing? (so i can answer)