There is a second definition of "boult" needed, as a verb, meaning to sift.
The example "“The context shows that the poet thinks his sudden side-issue not trivial or tedious, but quite the contrary, he quits it only because he cannot "boult it to the bren", i.e., sift it down, analyze it satisfactorily.” The Catholic Encyclopedia, Volume 3: Brownson-Clairvaux"
illustrates this other meaning. See "boulting". It is term used in milling and baking, relating to "sift", as this example clearly shows.