Jonson's poem is obsessed with measuring things, both literally and metaphorically. There are plenty of references to the speed of Echo's song ("slow," "slower "), for example. Plus, there's the weight of Echo's music ("List to the heavy part the music bears ") as well as the remark that "Woe weeps out her division," a word that refers to a section – a measured piece – of music. What's this preoccupation of measurement all about? Well, the more grief Echo can quantify, the more believable her love for Narcissus, right?
Line 1: Echo tells the "fount" to "keep time" with her "salt tears." This apostrophe tells us that Echo feels a strong connection to nature, and also that she sees a rhythm in it. Nature, like people, can keep time.
Line 3: Echo tells the springs to listen to the "heavy part" of the music. Heavy part is a metaphor for the grievous tone of Echo's song, and she wants to make sure the spring knows just how awful she feels.
Line 4: Echo tells us that "woe weeps out her division." A division is a section in a piece of music, which makes it yet another form of measurement in the poem. The repetition of the letter "w" in "woe weeps" and the "s" in "she sings" is called alliteration, which adds to the sense of rhythm in the line.