46 



Life and Letters of Francis Galton 



practiciil photographic sclieine was then evolved'. The difficulty of hi-projec- 

 tion met Galton at a very early stage of his nieteorologiciil work, and he 

 solved it by the construction of a compound pantagraph. He gave a great 

 deul of thought to the subject, and his pa|>er8 contain numerous devices and 

 suggestions for an instrument of this character. His work upon it Ijegan in 

 1867, but was not ccmipleted till 1869, when the first compound pantagraph 

 constructed by Mr ('. Beck was placed in the Meteorological Office. 



The general idea is that of a double or compound pantagraph. The 

 tracing pointer has a horizontal and a vertical motion. Tne former is con- 

 veyed through one pantagraph to the drawing-board on which the paper for 

 reproduction is set ancj the latter through a second pantagraph to tlie re- 

 producing pencil or style. The design on page 45 — not the one finally 

 adopted — in Galton's autograph indicates his ideas. The two pantagraphic 

 linkages uACBb and a'A'C'B'h' are the fundamental features. C is a fixed 

 pivot which may be either in AB or in a continuation of the rod AB; then 

 if the lengths of a^, CA, CB and Bh be so adjusted that the triangles CAa 

 and CBh are similar for any one position of those triangles, they will Ixj 

 similar for all positions, and consequently the distances a and h move in 

 their constrained horizontal paths will be in the adjustable ratio of CA to CB', 



(ialton'R doable or drill panUijnipli. 



' Wo shall see liiU-r that (JalUm actually solved the problem, but did not publish his solution. 

 A simple 'biprojector' was mode for the writ<T by Mr Horace Darwin many years ago 

 for drawing paral)olae, ellipses, probability curves, etc., but it involved the cutting of a definite 

 metal tetnplaU,' for each type of curve; it might possibly be adapt<>d for reducing drawings. 



* This fundamental principle of Galton's compound pantagraph is discussed by him with 

 proof in a letter of July 15, 1869, but he was inquiring for a maker even in May of 1869. 



