Allen and Greenough's New Latin Grammar, section 395

Heavy Construction

The Allen and Greenough is still under construction; so some links may not work quite the way you would expect.

395.

Transitive verbs compounded with prepositions sometimes take (in addition to the direct object) a Secondary Object, originally governed by the preposition: -

Caesar Germanos flúmen traicit (B. C. 1.88), Caesar throws the Germans across the river.
idem iús iúrandum adigit Afránium (id. 1.76), he exacts the same oath from Afranius.
quós Pompêius omnia sua praesidia circumdúxit (id. iii. 61), whom Pompey conducted through all his garrison.

NOTE 1: This construction is common only with trádúcó, tráició, and tránsportó. The preposition is sometimes repeated with compounds of tráns, and usually with compounds of the other prepositions. The ablative is also used: -

dónec rés suás tráns Halyn flúmen tráicerent (Liv. xxxviii. 25), till they should get their possessions across the river Halys.
(exercitus) Padó tráiectus Cremónam (id. xxi. 56), the army was conveyed across the Po to Cremona (by way of the Po, §429. a).

NOTE 2: The secondary object may be retained with a passive verb: as, - Belgae Rh=enum traducti sunt (B. G. ii. 4) the Belgians were led over the Rhine.

NOTE 3: The double construction indicated in § 395 is possible only when the force of the preposition and the force of the verb are each distinctly felt in the compound, the verb governing the direct, and the preposition the secondary object.

But often the two parts of the compound become closely united to form a transitive verb of simple meaning. In this case the compound verb is transitive solely by virtue of its prepositional part and can have but one accusative, - the same which was formerly the secondary object, but which now becomes the direct. So tr=aici=o comes to mean either (1) to pierce (anybody) [by hurling] or (2) to cross (a river etc.):

gladió bominem tráiécit, he pierced the man with a sword. [Here iació has lost all transitive force, and serves simply to give the force of a verb to the meaning of tráns, and to tell the manner of the act.]
Rhodanum tráiécit, he crossed the Rhone. [Here iació has become simply a verb of motion, and tráició is hardly distinguishable from tránseó.]

In these examples hominem and Rhodanum, which would be secondary objects if tráiécit were used in its primary signification, have become the direct objects. Hence in the passive construction they become the subjects and are put in the nominative: -

homó tráiectus est gladió, the man was pierced with a sword.
Rhodanus tráiectus est, the Rhone was crossed.

The poetical tráiectus lóra (Aen. ii. 273), pierced with thongs, comes from a mixture of two constructions: (1) eum tritiecit lóra, he rove thongs through him,[1][Perhaps not found in the active but cf. tráiecto fúne (Aen. v.488).] and (2) eum tráiécit lóris, he pierced him with thongs. In putting the sentence into a passive form, the direct object of the former (lóra) is irregularly kept, and the direct object of the latter (eum) is made the subject.