[ 801 ] 
to this, add the mean diameter of the columns, 3 feet 
4I inches, their fum, 13 6 feet 7-f inches, equal to 
1 455 3 3,6 London parts, is the diameter of the circle 
paffing through the centers of the columns j which, 
divided by 150 (as round a number as can be defired), 
gives 5)70,2 fuch parts for the mealure of the Roman 
foot. 
The mean depth of the four long chapels, be- 
tween the fhafts of the columns and the wall, is 
9 feet 1 inch, = 9764,3 ; which, divided by 10, 
gives a foot of 97634 London parts ; but by io|, one 
of 952,6 ; which is certainly too fmall. And if this 
was an arbitrary meafure, we cannot admit the frac- 
tion of a palm in it. Therefore, we may fuppofe 
thefe chapels were intended to be 10 Roman feet 
deep in the clear between the columns and the wall. 
The mean diameter of the columns, 3 feet 
4-f inches, = 3622,3d London parts, divided by 3|, 
gives a foot of 966 parts. The fhafts of thefe co- 
lumns meafure 27 feet i| inches, whofe eighth 
part (reckoning the fhaft at 16 modules) is 3 feet 
4! inches ; which is fomewhat lefs than the mean 
of their diameters. Yet even this meafure (which is 
equal to 3610,5 London parts) does not want half a 
Roman digit of 3| feet of 970,2 parts each j which 
the architect might fafely negledt, and allow 3I feet 
for thefe diameters, in his general plan. 
Now, if to twice the depth of thefe chapels, or 
20 Roman feet, we add twice the femidiameter of 
the columns, or 3-| fuch feet ; and to their fum, the 
diameter of the circle paffing tnrough the centers of 
the columns, 150 feet; we fhall have 173I Roman 
6 
