(A), in workman's language, means a portion of time allotted to some particular work, and from which the men are relieved when the limited time expires.
To spell is to relieve another at his work.
Spell ho! An exclamation to signify that the allotted time has expired, and men are to be relieved by another set.
A pretty good spell. A long bout or pull, as a “spell at the capstan,” etc. (The German spicl means a performance as well as a play, game, or sport.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894