10.38

In answer to the previous letter. Build the aqueduct, but find out who wasted money on the old one.

curandum est: “it must be attended to,” "steps must be taken" (Radice).

eā … dīligentiā: ablative of manner. 

quā dēbēbis: “with which you ought” (referring to the degree of diligence).

adgressūrum: modifying tē. Supply esse in indirect statement after crēdō.

medius fīdius: a typical Roman oath, “I swear, by the gods.”

pertinet: "it pertains to," + ad + accusative to indicate responsibility or relevance.

quōrum vitiō: “by whose fault,” i.e. whose fault it was.  

nē, dum inter sē grātificantur, et incohāverint aquae ductūs et relīquerint: “that they may not go on beginning and abandoning aqueducts with mutual complaisance.” The implication is that the appropriations were managed with the purpose of diverting the funds into the pockets of officials and contractors, who purchased mutual concealment by sharing the spoils (Merrill).

inchoāverint ... relīquerint: note the tense (Kingery).

quid compereris: pf. subj. in an indirect question after pefer in nōtitiam meam: "what you have found out."

This sounds like a personal note from Trajan who has not read Pliny’s letter carefully. Its main point was the request for an engineer (Radice).