Question: where does the city of ember get its light from

title: The City of Ember
text: As the book stats off a coalition of architects , scientists , and doctors known as `` The Builders '' have assembled Ember , a subterranean city with supplies for its inhabitants to survive at least 200 years , to elude an impending disaster . They handed the first mayor of the city a locked box , holding instructions for the city 's inhabitants , to be passed down from one mayor to the next . This passage continues until the seventh mayor who , in search of a cure for a deadly cough tries multiple times to break it open but fails each time , then dies before he can return the box to its rightful place or inform anyone else of its importance . Two hundred forty - one years after Ember is established , the city ′ s supplies are in danger of exhaustion and its hydroelectric generator is in decay . At their graduation ceremony , young people are assigned their jobs : Lina Mayfleet is assigned the job of `` Pipeworks Laborer '' and Doon Harrow is given the job of `` Messenger . '' However , both are displeased with their assigned jobs so they exchange their assignments in secret . Lina 's younger sister , Poppy , exposes the instructions left by the Builders , but in teething leaves them almost illegible . Later , Lina asks Doon to help her reconstruct the paper . After much trial and error , she and Doon decipher the instructions from the Builders , which inform them of how to exit the city of Ember . Lina 's grandmother dies shortly afterward , and Lina and Poppy move in with a neighbor , Mrs. Murdo . At work , Doon discovers that the mayor of Ember and a storeroom worker named Looper have been stealing supplies , and he and Lina report the crime . Upon following the instructions given in the note , they discover boats and candles meant for use in the exodus . On their return to Ember , they learn that the mayor has declared them fugitives from the law . Lina is arrested and taken to the mayor , who threatens to throw her in jail . Suddenly , a blackout occurs and allows her to escape without being seen . Lina , Doon , and Poppy escape in a boat through the river that supplies Ember 's electricity . When the boat stops , they learn the origin of Ember from a diary left by one of its original colonists . Shortly after they are faced with a very steep climb and emerge onto the surface where they see their city from above and realize for the first time that Ember is underground . They throw a rock with instructions tied to it down to the city in hope that the people of Ember will escape . The novel ends with Mrs. Murdo finding the message on the street . 

Answer: answer: The City of Ember has a river that supplies Ember's electricity with a  hydroelectric generator.