Metrum 1

Meter: Elegiac couplets. See on 1M1.

line 1

1: Lines 1-8: The Tigris and Euphrates rivers arise from a single source (this was a common belief in antiquity); there is an order in nature that keeps them from joining again (an event whose results would be chaotic). (For the phenomenon in lines 1-2 cf. English "Parthian shot.")
Rupis: genitive singular.
Achaemeniae: "Persian," (Achaemenes was the first Persian king).
uersa: accusative modifying spicula.

line 2

pugna fugax: = pugnatores fugaces.

line 3

se: se . . . resoluunt: "free themselves, get loose."
resoluunt: se . . . resoluunt: "free themselves, get loose."

line 4

abiunctis: abiunctis . . . aquis: ablative absolute.
aquis: abiunctis . . . aquis: ablative absolute.

line 6

confluat: sc. id (explained by the quod-relative clause that follows.)
alterni: "of each," the Tigris and the Euphrates.
quod: object of trahit.

line 7

conuenient: future indicative in a mixed conditional.
uulsi: participle < uello, "tear up, uproot."

line 8

modos: here (often in later Latin), "appearances."

line 9

quos: quos . . . uagos . . . casus: object of regit. Quos is connecting relative ("and these").
uagos: quos . . . uagos . . . casus: object of regit. Quos is connecting relative ("and these").
terrae decliuia: terrae decliuia (< decliue, "slope, incline") . . . gurgitis et . . . ordo: compound subject of regit (singular by agreement with the nearest element of the subject).
casus: quos . . . uagos . . . casus: object of regit. Quos is connecting relative ("and these").

line 10

gurgitis et: terrae decliuia (< decliue, "slope, incline") . . . gurgitis et . . . ordo: compound subject of regit (singular by agreement with the nearest element of the subject).
ordo: terrae decliuia (< decliue, "slope, incline") . . . gurgitis et . . . ordo: compound subject of regit (singular by agreement with the nearest element of the subject).

line 11

permissis: permissis . . . habenis: ablative absolute, "with the reins allowed [to slip], unchecked."
fluitare: "to move unsteadily, wobble."
habenis: permissis . . . habenis: ablative absolute, "with the reins allowed [to slip], unchecked."

line 12

ipsa: sc. fors.