As in “I found the words to every thought” where there was “but One” thought she could not put words to, and here there is “but one” phrase she has not “heard”. Yet unlike the previous poem, she seems to actually be describing this phrase and even wants it hear “it again” by the end of the poem.
What is most remarkable about this poem is how you can hear what she is describing – the sounds of nature are embedded in the “phrase” of the “English language”. For example, the ‘lo’, ‘a’ and “la” sounds as they flow through “Low as the laughter” (with that ‘s’ sound in the middle) is reminiscent of the sound of a “Cricket”. She repeats this again in “Thunder’s Tongue” with the sharp ‘t’ sounds and how the two syllables of “Thunder’s” is resolved with the single syllable blast of “Tongue”. The word “Tongue” is also doing double duty in that visually it reminds us is a serpents “Tongue” of lightening during a thunder storm thus she is combining the audible and visual elements into the words of the “English Language”.
The second stanza hints at how we often don’t even notice how language is working on us: it’s a subtle influencer that works more in our subconscious than it does when we are awake. For example, she mentions the “Whippowill” and rhymes this with “a’lull” which alludes to sleep and dreams. The whippoorwill is a nocturnal bird and so as we sleep its language enters our dreams and so Emily might be saying that the language of nature can be heard and understood best when we are asleep as it will influence our dreams, perhaps even a dream of a “Caspian [Chior]” “mummering” like the tide in the middle of the night. This stanza has a dreamlike quality to it and I believe she is suggesting that it is in dreams when we can most clearly hear the langauge of nature.
The third stanza seems to support this hypothesis of nature’s language speaking to us through our own language – just as if we too were like the “Whippowill” and the “English Language” was no different than birdsong. She uses the word “Orthography”, which is “a system of spelling or notation” (OED) and in the second line of this stanza describes her “simple sleep” in which this grammar of nature is “Breaking in bright” into her dreams. She continues with the word “Prosepctive” which is “a device which allows one to see objects or events not immediately present” (OED) thus further suggesting that only when one is dreaming can we truly interpret the language and grammar of nature. And yet, once she wakes, she weeps because she is no longer in a state which will allow her to interpret this language. Even the sound of the word “Prospective” (aside from its relation to the word perspective) has the quality of a distant, rumbling thunder leftover from the first stanza.
Thus the final stanza expresses her wish to hear “it again” and what it is she wants to hear can only be heard either in dreams or obtusely through the sounds of words in the English language. This whole poem is her attempt to mimic the sounds of nature inside her limited Anglo-“Saxon” language and her desire to continue to hear it. Ironically, she ends the poem with the word “Hush” as if she wants silence but is asking to actually hear something. Normally one would say ‘speak up’, not “hush” if you want to hear something clearer, so perhaps she is suggesting that what she wants to hear can only be heard in the silence – perhaps she’s even suggesting that words as they are printed on the page are silent until we speak them and so we are like Emily whom the words speak “Only to me” when we read the poem and thus can hear so much more inside this silence, like a dreamer who when we watch them sleep quietly but inside their mind a thunderstorm rages and the “Whippowil” inspires visions of “Caspian Choirs”.