[ 802 ] 
feet for the diameter from wall to wall of the op- 
polite chapels. 
Defgodetz has given four meafures of this dia- 
meter, & whofe mean, 1 58 feet af inches, is equal to 
168570 London parts, which, divided by 1 7 3 - j , 
give 970,2 — , for the meafure of the Roman foot; 
agreeing with that found from the diameter of the 
circle palling through the centers of the columns. 
Or if, inftead of taking a mean of the four dia- 
meters from wall to wall, we add twice the mean 
depth of thefe chapels, and twice the mean femi- 
diameter of the columns, to the mean diameter of 
the circle palling through their centers, we lhall have 
158 feet 4 inches for the diameter from wall to wall, 
equal to 168688 London parts; which, divided by 
17 3JL, give 970,9, for the meafure of the foot. 
But if this diameter from wall to wall was a prime 
meafure, as well as that of the circle palling thiough 
the centers of the columns, the depth of thefe chapels 
mull have been a refiduary meafure, and might be 
more or lefs than 10 Roman feet. And if we divide 
the diameter laft mentioned by 174 (which being 
compofed of 150, and twice 12, may here be rec- 
koned a round number), it will give a foot of 969-?- 
London parts. But if, inllead of twice the mean 
depth of thefe chapels, we take twice the greatell 
depth, 1 8 feet 6 inches, for part of this diameter, 
the fame divifor will give a foot of 970^ hich parts. 
The meafure of the diameter from out to out can- 
not be fo well determined as either of the foimci ; 
for Defgodetz has given but one meafure of the outer 
wall, which is 5 feet 44 inches ; to this we mull add 
i foot io-j inches, for the mean depth of the niches 
m 
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