distance. O B D is a light bar passing freely through B and forming
one side of a parallel ruler of two or more points, g g, h h, i i.
Along i i is a slot and in this works a loaded block containing a
wheel P', whose plane is always parallel to i i. This block also
passes through a slot in D E, an arm at right angles to B C. A little
consideration will show that P', if worked at all, would trace out the
sum curve of P.

It was only when I showed the rough idea of this to Professor
Kennedy, with the view of ascertaining what would be the amount of
back-lash and friction, that I learned that Mr. Boys had already
invented a very similar integrator. In his model the double parallel
ruler is replaced by two endless strings and pulleys, and the bar, B
C, by a T square.

Although this integrator was afterward made in a less crude form, I do
not think it has ever been a practical instrument for the draughtsman.
Shortly afterward I came across a work by Abdank-Abakanowicz, entitled
"Les Integraphes," being a study of a "new kind of mechanical
integrator."

The new kind of integrator was really only an independent version of
Boys' instrument, but in many respects a great improvement. The real
merit will ultimately belong to the scientific instrument maker who
constructs an instrument reasonably cheap and capable of efficient
practical service. Abdank-Abakanowicz's integrator however certainly
went further in the practical direction than any previously
constructed. The drawing board machines, it is true, of rather a
complex nature, were actually exhibited to the Paris Academy, but no
more have been made. The instrument before me was made by Coradi, of