48 Life and Letters of Francis Galton
accompanying figure. Galton constructed his surface from a table of 400 values of the vapour-tension, 4(i0 holes being bored into a solid rectangular block to these 400 values spaced properly apart, and then the remainder cut away, filed and smoothed. The construction at that time did not cost more than £6. Here again it is easy to think of many purposes to which a machine of this kind could be put, but as it has never been made as a commercial article, it has never come into general use. Perhaps this brief notice may remind investigators of the existence of Galton's design.
Galton's Trace Computer-a machine for tracing a curve, whose ordinate is any arbitrary function
of two other variate values at the same abscissa or time.
A third instrument designed by Galton a little earlier (1867) never came in being, owing probably to a discouraging letter from Balfour Stewart at that time at the Kew Observatory, who laid great stress on comparison of pairs of automatic meteorological records at different intervals. Galton was easily discouraged and was apt to treat the judgments of the really able people whom he consulted as sure to be better than his own. It certainly was a pity that in this case he was put off completing his model. It was of the following nature : a map is mounted horizontally on, let us say, a metal plate; then holes are drilled at each meteorological station, and a rod of a convenient length is free to move vertically up and down in the hole. Templates are now cut to the continuous automatic records of any meteorological character for these stations and are fixed in vertical planes