She went as quiet as the Dew — Dew-Snail

Urbain Le Verrier had predicted the existence of Neptune by mathematics. Dickinson was comparing herself with him in predicting the route of a dew-snail.

She went as quiet as the Dew 1
From an Accustomed flower.
Not like the Dew, did she return
At the Accustomed hour!

She dropt as softly as a star 5
From out my summer's Eve-
Less skillful than Le Verriere
It's sorer to believe!
(F.159/J.149)
[1] She:: a dew-snail; snail, a slow or indolent person; a sluggard (OED 2c). Dew:: a hint on dew-snail.
[3, 5] Not, return, dropt:: the snail dropped from a beam.
[4] Accustomed hour:: the usual time that dew appears; and the appearing of Neptune predicted by Le Verrier.
[6] summer:: a beam for supporting. eve:: a hen-roost (OED n.2).
[7] Le Verriere:: Urbain Le Verrier (1811-1877), a nineteenth century French mathematician, who had predicted the discovery of Neptune by calculation. His skill in climbing was bad, supposedly, comparing with a snail.
[7, 8] Less skillful, sorer:: Dickinson was failed to predict the appearing of the dew-snail.

That he could couch a dew-snail's eye
Or shoe a gnat, or geld a fly,
With any Ashton Lever. ─ The Monthly Review (1785)