654 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
necessarily correct. The mere establishment of the camp was suffi- 
cient for the purposes of the moment, and to fortify it permanently 
at once would have been to further alarm the people of Rome without 
adequate reason. The coin, too, of Claudius on which the camp is 
pictured appears to show a.timber fence rather than a brick wall. On 
the whole, it seems quite likely that the wall was not built before the 
time of Claudius, or even Nero. With regard to the other example 
also, the works are immense, and if completed by Caligula must have 
been built in three and a half years at most. There is nothing im- 
probable in supposing that part of the palace to have been completed 
by Claudius; and there is a distinct difference of system between the 
lower and upper parts of the work, the lower being of opus reticulatum 
with brick quoins and arches only. One part of the ruins of Nero’s 
“ Golden House” Doctor Middleton also describes as being faced 
with brick, and another with a mixture of opus reticulatum and 
brick —that is no doubt with brick quoins. It would be rash to 
entertain a positive conviction opposed to the generally accepted 
opinion, but it seems possible we may eventually discover that though 
fired building bricks were gradually coming to the front from the 
middle of the first century B. C., and may have been given a certain 
impetus by the metropolitan building act of Augustus, their use in 
the place of stone for entire wall facings did not become general 
before the time of Nero. If that should be so, we may perhaps also 
find that the system of building concrete vaults with a lacing or 
framework of brick did not reach its final development before the 
middle of the second century of our era. 
With respect to Roman concrete, I should like to enter a protest 
against the indiscriminate use of the term by Doctor Middleton and 
other writers, who apply it even to the walls called by Vitruvius in- 
certum and reticulatum, in opposition, as it seems to me, to what 
Vitruvius himself tells us. It may be merely a question of a defini- 
tion, but if a writer has one definition for a thing and his readers 
another, he becomes misleading. Doctor Middleton argues for the 
word concrete because “ the result was a perfectly coherent mass, like 
a block of stone, particularly unlike what is now usually known as 
rubble work.”’ But to the professional reader the difference between 
concrete and rubble work is not one of result, but of the way in which 
the result 1s obtained—the results by either method may differ as 
chalk from cheese. The doctor himself acknowledges that in the ex- 
amples he examined the larger stones were placed with so much regu- 
larity that they must have been thrown in separately. He arrived at 
the conclusion that thick layers of mortar, mixed with small stones, 
and layers of larger stones, were thrown in alternately.2- When this 
Remains, II, 148, 149. oTbid., Ey 47: 
