14 GALTON'S SUN SIGNALS FOR TRAVELLERS. [Nov. 28, 1859- 



Exhibitions. — Sketches in Burmah, Punjab, Himalayas, and Kash- 

 mir, by Captain H. H. Godwin Austen, f.r.g.s. ; Sketches of the 

 Scenery, &c., on the Zambesi, by Thomas Baines, Esq., f.r.g.s. ; Pho- 

 tographs of Ancient Hindoo Temples, by W. H. Purdon, Esq., 

 F.R.G.S. ; Sonnenstem's Map of Central America, &c., were exhibited. 



The Papers read were : — 



1. Sun Signals for the Use of Travellers (Hand Heliostat). By Francis 

 Galton, Esq., f.r.g.s. 



A PAPER was read by Mr. Galton in 1858 before the British Associa- 

 tion on the principle, explained by a rough wooden model, of the 

 instrument which is the subject of his present communication, but 

 which, in the interim, has been considerably modified and improved. 

 Many matters connected with its use and application have been more 

 thoroughly worked out. The following is an abstract of that part 

 of the paper which bears more immediately on the construction of 

 the hand heliostat. Instruments made for the author by Messrs. 

 Troughton and Simms, were laid on the table. 



The principle of the instrument may be shortly recapitulated as 

 follows : — The fact is a well known one, that if a looking glass be 

 held in such a manner that a distant observer can see a portion of 

 the sun's disc reflected in it, it assumes the appearance of a brilliant 

 star, and can be seen at extraordinary distances. Many endeavours 

 have been made, with various success^ to utilise this remarkable 

 power for the purpose of telegraphs, — the signals being distinguished 

 by diiferent combinations of flashes, — but no instrument has hitherto 

 been contrived that admits of being carried on the person, held in 

 the hand, and used at will. From experiments detailed by the 

 author, it appears that a mirror whose rays are obstructed by a 

 screen having an aperture of only -i of an inch in the side, is 

 visible to the naked eye at a distance of ten miles, if the background 

 be dull and the air perfectly clear. In other words, a mirror is 

 visible, under those circumstances, if its effective area is a square 

 whose sides subtend one tenth of a second of a degree in angular 

 measurement. But, in practice, some allowance has usually to be 

 made for the opacity of the air traversed by the rays, and a simple 

 formula helps us to collate experiments, made under different 

 circumstances of distance and fine weather. It is necessary in each 

 particular case to make the best estimate possible of the extreme 

 distance which any reflected solar rays could possibly traverse; 

 call this d (the distance at which high lands cease to be visible at 

 the time of experiment gives a good clue towards estimating d). Let 

 X, less than d, be the distance of the signaller. Then, on a perfectly 



