sentences:  Earlier, the boys had written a letter to a nearby university professor inviting him to come and give a lecture to their young astronomers' society.The letter arrives while the professor is away and is received by his ambitious young assistant, who comes to Pacific Grove to give the lecture in the professor's stead.The young assistant, Horatio Peabody, ends up going to the Mushroom Planet as a stowaway, and causing quite a bit of trouble there.