Noun, not verb
Sheafs is here a plural noun, not a singular verb. There does exist a verb sheaf meaning to bind up into a sheaf or sheaves, but that is not what this instance of the word is.
And you’re right that sheaf has as its oldest sense a bundle of things, usually grains. It appears metaphorically in the familiar Gospel tune “Bringing In the Sheaves”.
What we have here in your quotation is another extended meaning. The OED gives these pertinent senses:
1e. A cluster of jets of fire or water darting up together. (Cf. F. gerbe.)
- 1811 Pinkerton Petral. II. 535 — A thousand sheaves of fire blew up into the air, where, breaking and dispersing, they fell like a shower of stars.
- 1857 Dufferin Lett. High Lat. 125 — A shining liquid column, or rather a sheaf of columns wreathed in robes of vapour, sprung into the air.
But even more likely is this one:
6a. Physics and Math. A bundle of rays, lines, etc. all passing through a given point.
- 1863 Tyndall Heat ix. §359. 302 — A sheaf of calorific rays.
- 1885 Leudesdorf Cremona’s Proj. Geom. 22 — A sheaf (sheaf of planes, sheaf of lines) is a figure made up of planes or straight lines, all of which pass through a given point (the centre of the sheaf).
- 1890 Eagles Descript. Geom. 303 — Sheafs of rays which in the case of a source of light..form a cone of which that source is the vortex.
Sheafs or Sheaves?
As for the variant inflections, sheafs and sheaves alternate. Both are current; I don’t know their relative popularity.
The sheaves form dates from the 16th century, and sheafs from the 17th.
In the north of Britain (northern England and Scotland) during the 19th century and perhaps later could be found dialectal plurals like shoves and shoofs.