I have endeavored to obviate this by always drawing the second sum
curve on a thin piece of paper pinned to the board, which can then be
moved back to the position of the first primitive. But this shifting,
of course, involves additional labor, and is also a source of error.
I should like to see the trace and guide chariots on the same line of
rails, one below the other, were this possible without producing the
bad effect of a skew, pull or push.
4. The practical integraph must not have a greater maximum error than
2 per cent. The mathematical calculations, which are correct to five
or six places of decimals, are only a source of danger to the
practical calculator of stresses and strains. They tend to disguise
the important fact that he cannot possibly know the properties of the
material within 2 per cent. error, and therefore there is not only a
waste of time, but a false feeling of accuracy engendered by human and
mechanical calculation which is over-refined for technical purposes.
For comparative purposes I have measured the areas of circles of 1
inch, 2 inches, and 3 inches radius, the guide being taken round the
circumference by means of a "control lineal," first with an ordinary
Amsler's planimeter and then with the integraph.
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