1. The price must be such that it is within the reach of the ordinary
draughtsman's pocket. The Amsler's planimeter at £2 10s. or £3 may be
said to satisfy this first condition. The price for the first complex
integraph designed by Coradi was £24 to £30. The modified form in
which I show it to-night is estimated to cost retail £14. Till an
equally efficient instrument can be produced for £5 I shall not
consider the price practical. If the error of its reading be not
sensibly greater than that of a planimeter, it is certainly worth
double the money.

2. The instrument must not be liable to get out of order by fair
handling and a reasonable amount of wear and tear. I cannot speak at
present with certainty as to how far our integraph satisfies this
condition; it is rather too complex to quite win my confidence in this
respect.

3. It must be capable of being used on the ordinary drawing board, and
of having a fairly wide range on it, i.e., it must not be limited to
working where the primitive is at one part only of the board.

This condition takes out of every day practical drawing use the
integraph invented by Professors James and Sir William Thomson, in
which the sum curve is drawn on a revolving cylinder. It is essential
that the sum curve should be drawn on the board not far from the
primitive, and that this sum curve can be summed once or twice again
without difficulty. The time involved in drawing the four sum curves,
for example, required in passing from the load curve to the deflection
curve of a simple beam, if these curves were drawn on different pieces
of paper and had to be shifted on and off cylinders, would probably be