Method of tracing Oval Arches. _ 303 
rivers.” We would add that granite and many other rocks are al- 
ways found rounded as well as greenstone but they are not at pre- 
sent always smooth. “This is owing to their surface having been dis- 
integrated and left rough by the action of the weather, while green- 
stone, which is firmer in its texture, has not been operated upon by 
this cause. It is true that they occur most abundantly in water 
courses, but they are also to be found in great numbers on the hills 
as has already been explained. No one can suppose that the boul- 
ders and gravel which now line the beds and banks of the rivers in 
Ohio could have been rounded and smoothed by the action -of the 
currents of those rivers. 
In a letter accompanying the above notice it is observed dt the 
expression primitive boulders commonly employed does not accu- 
rately describe the fact, since some of these boulders are secondary. 
There is an interesting connexion between the boulders, and the 
diluvion which has sometimes been overlooked. For example, at 
‘the bluffs, a few miles below Circleville, in the various and very cu- 
rious windings and turnings of the different layers of clay, sand and 
gravel, large quantities of boulders‘have been found by digging, 
and the gravel is nothing more than boulders on a . 
Is not this conclusive as to the origin of the boulders? z 
Arr. VIII.—On the method of tracing oval arches from several cen- 
tres; by Epvwarp Miuier, A. M. Civil Engineer. 
‘Tue difficulty of describing an Elliptical arch of great span, 
taakes it important to find a curve, which, resembling the Ellipse in 
its graceful and convenient form, may be constructed with great- 
‘er ease and accuracy. Oval arches are frequently formed by three 
arcs of circles; the radii of two being equal, and their centres situa- 
ted in the transverse axis; while the middle are has a longer radius, 
and the centre in the line of the conjugate. 
Several methods of tracing ovals from thrée centres are given in 
the elementary works on Carpentry, &c. But when the difference 
between the rise and half-span is considerable, the change of curva- 
ture becomes very perceptible and unpleasant to the eye, and it is 
advisable to employ a greater number. This has been done by the 
rench in the construction of some of their finest bridges. A beau- 
tiful example is Neuilly, built by Perronet, and traced from eleven. 
