HISTORICAL INTRODUCTION. 3 

saw and measured them, or to reconcile the facts observed with any of the 
eories suggested, I decided to make an independent examination of the 
entire building and of the evidences bearing upon its history. In this exam- 
ination my purpose was twofold: first, to prepare a more exact plan of the 
imperial Atrium, so far as it was then feasible, into which the walls discovered 
since 1889 should be incorporated as soon as they should be made accessible 
through their official publication;' and, second, to reconstruct the archi- 
tectural history of the building, including, so far as possible, that of the 
republican structure beneath it. 
The carrying out of the former purpose was a simple matter, though 
tedious, consisting merely in the careful measurement of such of the walls 
now standing as have been published. In the plans based upon these meas- 
urements, which are here presented,” a number of errors in the earlier plans 
have been corrected and some details of importance, as I hope, added. 
The reconstruction of the architectural history of the Atrium was, how- 
ever, less simple, involving, as it did, the determination of the structural 
units composing the building and their chronological relation to one another, 
as well as of the periods to which they are to be assigned. For the determi- 
nation of the various units and their relation to one another, a careful exam- 
ination was made of the building as a whole, as well as of the individual walls 
of the various parts; in this examination special consideration was given to 
the following points: 
1. The comparative level of the individual walls in each part and the 
relation, with respect to level, between the various parts.° 
2. The unity, in the several parts, in architectural plan and in structure, 
the latter as shown especially by continuity in brickwork and 
concrete. 
3. The superimposition of walls of one type upon those of another. 
4. The methods of construction, that is, the thickness of the individual 
walls and the occurrence and frequency, in them, of bonding-courses 
composed of large square bricks, the tegule bipedales of Vitruvius;* 
and where it could be ascertained the depth of the concrete founda- 
tions and the width of the courses of brick and the layers of mortar.° 

1 By the courtesy of Commendatore Boni, Director ‘In earlier investigations concerning brickwork, 
of the Excavations, I continued my work in the width of a course of bricks and a layer 
the Atrium during the progress of the exca- of mortar together has been regarded as a 
vations. I was not, however, allowed to unit of measurement, or the number of 
measure the new walls. courses of brick to the meter has been 
2 Plans A-F. reckoned. Both of these methods are un- 
3 See plate m1, fig. 1. reliable, since with the decrease in the width 
4De Arch., vil, 1, 73 4,2. The tegule bipedales of the bricks in the later periods there is a 
Lanciani (R. and E., 47) holds appear first 
in the Pantheon and Mausoleum of Hadrian. 
They are used, however, much earlier, 
being found occasionally in the walls of 
Nero, and regularly in those of Domitian. 
corresponding increase in the thickness of 
the mortar. The width of the two together, 
therefore, and the number of courses of 
bricks in a meter remain in general the same 
(see plate 11, fig. 2). 
