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Transition Studies   47

Perhaps a simpler arrangement would be to replace the linkage by pins at a and b running in a slotted bar turning about a pivot C. We reproduce (page 46) an illustration of the final apparatus; the pencil or style could be replaced by a drill'. With this instrument for twelve years the continuous automatic weather records for seven stitions (velocity and direction of wind, dry and wet bulb thermometers, barometer, vapour-tension and rain) were reduced to manageable dimensions and published. Of this publication Galton remarks

"It surprises me that meteorologists have not made much more use than they have of these comprehensive volumes. But there is no foretelling what aspect of meteorology will be taken up by the very few earnest and capable men who work at it. Each of them wants voluminous

data arranged in the form most convenient for his own particular inquirya."

Probably the use has not been made of these graphical charts that might well have been made; but Galton's own results indicate that we need simultaneous data for a far wider range than Great Britain, and further modern methods of multiple correlation, which seem likely to be most productive of result in present day meteorology, demand numerical values, and these are hard to obtain from the graphs; not only can they scarcely be read off with the requisite accuracy, but to reconvert the graphs into any numbers whatever is in itself a most arduous task.

Galton's compound- pantagraph has indeed a far wider field of usefulness than reducing automatic weather returns. The difficulty is that it is not made commercially and procurable at a moderate cost.

A second - instrument devised by Galton about this time will be found described in the Report of the Meteorological Committee, 1871 (p. 30). It was devised for obtaining mechanically the vapour-tension curve from the curves of dry and wet bulb thermometers, but again it can be used to serve a much more general purpose, namely to obtain the curve of a variate whose ordinate is a given function of the ordinates of two other curves-all three curves having the same abscissa. The machine depends upon the construction of a surface corresponding to the function the variate is of the two other ordinates (i.e. in Galton's case the vapour-tension in terms of wet and dry bulb thermometer readings). By fine screw adjustment the cross-hairs in two microscopes are brought into accordance with the tops of the ordinates in the two curves, but the screw which adjusts one microscope moves the surface parallel to one axis, and the screw which adjusts the other microscope moves the surface perpendicular to this direction. Thus a vertical style resting on the surface raises to an adequate height a scriber which marks the ordinate or function-value of the compound variate3. It would be out of place here to give a more complete account of the instrument, but my more mechanically minded readers will grasp the general idea from the

' The theory is fully described in the Minutes of the Meteorological Committee, 1869, p. 9. It is also figured in the Katalog mathematischer Modelle, Apparate and Instrumente, of the Deutsche Mathematiker-Vereinigung, 1892, p. 232.   2 Memories, p. 236.

s In Galton's actual instrument (see our p. 48) the required curve was recorded on a zinc plate (partly removed in figure to show scriber R). The scriber received when adjusted a blow from the hammer H worked by the action of the operator's foot on a treadle.