What does 'keep your cool' mean?

Which part of this excerpt from john keats' poem "endymion" contains a simile? therefore, 'tis with full happiness that i will trace the story of endymion. the very music of the name has gone into my being, and each pleasant scene is growing fresh before me as the green of our own vallies: so i will begin now while i cannot hear the city's din; now while the early budders are just new, and run in mazes of the youngest hue about old forests; while the willow trails its delicate amber; and the dairy pails bring home increase of milk. and, as the year grows lush in juicy stalks, i'll smoothly steer my little boat, for many quiet hours, with streams that deepen freshly into bowers.