command of a squadron or a cohort with greater facility than in the
former times, they never obtained it without passing through a tolerably
long military service. Usually they served first in the praetorian
cohort, which was intrusted with the guard of the general: they were
received into the companionship (contubernium) of some superior officer,
and were there formed for duty. Thus Julius Caesar, though sprung from a
great family, served first as contubernalis under the praetor, M.
Thermus, and later under Servilius the Isaurian. (Suet. Jul. 2, 5. Plut.
in Par. p. 516. Ed. Froben.) The example of Horace, which Gibbon adduces
to prove that young knights were made tribunes immediately on entering
the service, proves nothing. In the first place, Horace was not a
knight; he was the son of a freedman of Venusia, in Apulia, who
exercised the humble office of coactor exauctionum, (collector of
payments at auctions.) (Sat. i. vi. 45, or 86.) Moreover, when the poet
was made tribune, Brutus, whose army was nearly entirely composed of
Orientals, gave this title to all the Romans of consideration who joined
him. The emperors were still less difficult in their choice; the number
of tribunes was augmented; the title and honors were conferred on
persons whom they wished to attack to the court. Augustus conferred on
the sons of senators, sometimes the tribunate, sometimes the command of
a squadron. Claudius gave to the knights who entered into the service,
first the command of a cohort of auxiliaries, later that of a squadron,
and at length, for the first time, the tribunate. (Suet in Claud. with
the notes of Ernesti.) The abuses that arose caused by the edict of
Hadrian, which fixed the age at which that honor could be attained.
(Spart. in Had. &c.) This edict was subsequently obeyed; for the emperor
Valerian, in a letter addressed to Mulvius Gallinnus, praetorian
praefect, excuses himself for having violated it in favor of the young
Probus afterwards emperor, on whom he had conferred the tribunate at an
earlier age on account of his rare talents. (Vopisc. in Prob. iv.)--W.
and G. Agricola, though already invested with the title of tribune, was
contubernalis in Britain with Suetonius Paulinus. Tac. Agr. v.--M.]

[Footnote 54: See Arrian's Tactics.]

The safety and honor of the empire was principally intrusted to the
legions, but the policy of Rome condescended to adopt every useful
instrument of war. Considerable levies were regul