Bernoulli's principle states that as water flows through a pipe from a high elevation reservoir to a lower elevation, its velocity increases to maintain the total energy of the water. Applying the Bernoulli equation between two points in the streamline allows calculating unknown parameters like velocity. In this example, the equation is simplified by eliminating terms that drop out, and used to calculate the velocity of water flowing from a 250m high reservoir, found to be 70 m/s.