368 Dr. Herschei/s Description of a 
the two sides of the sheet were gradually brought to take hold 
of each other. As we proceeded, the wedges within the arches 
were forced in successively, till at last, with much care and 
considerable difficulty, the two sides completely embraced one 
another, and were kept stretched by the swelled inside arches. 
Another circular arch, closed in with boards all around, well 
rounded off, and only about 2 feet 3 inches long, had a va- 
cancy at the top into which we could introduce the iron seam- 
ing bars, fig. 23. for indenting, and 24. for closing up the long 
seam of the two sides. This arch also had its stretchers for 
swelling it up, and served at the same time, as soon as the 
seam was properly closed, to beat with mallets the whole 
sheet all around upon its well-finished outside, in order to take 
away any accidental bulge which it might have received in 
the long preparations it had undergone, till it came to the 
present state. 
The same arch, as soon as any portion of the tube had been 
' done, was removed to another place, and the whole was by 
this means completely seamed up. 
The theory upon which the strength of so thin a cylinder of 
iron is founded, is, that the sides of it must unavoidably sup- 
port it, provided you can secure the cylindrical form of the 
tube. 
It appeared to me the most practical way to obtain this end 
by the following contrivance. By a few experiments I found 
that a slip of sheet iron, a little thicker than that of the tube, 
and doubled to an angle of about 40 degrees, as in fig. 28. 
might afterwards be made circular, as in fig. 29. The deepest 
we could conveniently bend, and such as I supposed would 
answer the end, was when the sides a b were about 2| inches 
