39 The plane figures with which we are concerned come mainly under the description of graphs of continuous variation.
1, 2, 3, 4 (plate) show enlargements from photo- General graphs by Hansky at Pulkowa (June 25, 1905); Appearance they are separated by intervals from 25 to 80 of photo- seconds, and he has succeeded in showing identity sphere.
Another useful set of graphs comprises those which give the relation between the expressions of a length, volume, &c., on different systems of measurement.
(C) Mensuration of Graphs Generally.
The second class of cases comprises equations involving two unknowns; here we have to deal with two graphs, and the solution of the equation is the determination of their common ordinates.