384 Mr. ludlam on the Engine 
fee the reafon; it would deftroy the oval firft begun to 
be turned, and form a new one in a different pofition. 
But there is another difficulty in turning ovals, efpe- 
cially fuch as have mouldings, as pidture-frames, See. 
The tool generally has all the mouldings formed upon 
it : now if it be laid flat upon the r&ft , and the engine fet 
to work; the mouldings will in fome places crofs the 
plane of the tool (or the top of the reft) at right angles 
(as in turning circles), in other places obliquely. This 
will make the feveral members of the mouldings leaner 
or fmaller in one part of the work than another. Nor 
will the cafe be altered if the mouldings be turned fepa- 
rately. Analogous to this, when an oval is drawn by 
the trammels, the line deferibed by the pencil will not, 
as in a circle, be always at right angles to the beam of 
the trammels. The oval line fo drawn will be at right 
angles to the deferibing beam, only at the extremity of 
the two principal axes where the beam coincides with 
thofe axes ; in all other places the oval line and beam 
make an oblique angle. It may be proper therefore to 
enquire how much this angle deviates from a right angle. 
This we fhall call the angle of deviation . 
All things as in fig. 1. draw the tangents tm and tn, 
to the point m in the ellipfe and the point n in the circle 
correfponding to each other; and from the nature of the 
ellipfe 
