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



tamed with perfect accuracy. Where a considerable depth, as well 

 as breadth, of landscape has to be searched, the operation is more 

 tedious. The landscape must be swept in closely parallel bands. 



This instrument is of course useless without sunshine, and is 

 intended chiefly for those lands and seasons where sunshine is the 

 rule and not the exception. It is believed that it would be of 

 constant sei-vice to a traveller in them. It requires no sky line, as 

 all other signals do, to bring it out into relief, but can be used from 

 any spot where the sun's rays reach it. It works in perfect secrecy 

 to all except those near the line of flash. Its power is enormous as 

 regards the distances across which it can communicate ; and lastly, 

 its portability is extreme. Fig. 2 can literally be carried in the 

 waistcoat pocket, and can make a signal visible to the naked eye, 

 under very adverse circumstances of haze and position of the sun, 

 at a distance of 5 or 10 miles. Instruments such as Fig. 3 would 

 probably be of great service to two or three travellers engaged in 

 triangulating a country, or to land parties communicating with a ship. 



Sir R. I. MuRCHisoN said they were much indebted to Mr. Galton for 

 calling the attention of the Society to this subject. Doubtless many of those 

 who were then assembled may be more gratified by descriptions of foreign 

 travel ; but the Society could not be too thankful to those who, from time to 

 time, refer back to the elements of the science and bring to its notice the con- 

 sideration of instruments of real value to the explorer. 



Sir Edward Belcher, f.r.g.s., regretted that General Portlock'or Colonel 

 James were not present to speak more decidedly on the use of the heliostadt 

 during the Trigonometrical Survey of Great Britain. But in the year 1835, 

 when he was engaged in the connection of the two surveys of Great Britain 

 and Ireland, for the object of completing the Hydrographic Survey of the Irish 

 Seas, he was informed, as the documents also witnessed, that one shot was 

 obtained from Slieve Donard, in Ireland, to Scawfell, in Cumberland, and vice 

 versdy a distance exceeding 108 miles, and that it was effected by heliostadts, 

 requiring fourteen days' close watching at each station where the parties were 

 encamped. 



In the year 1833 a complete set of instruments were supplied to him in order 

 to connect the vessel stationed, and moored in position, on the Skerki Reef, 

 with the j)ositions on shore, at Zembra at the mouth of the Bay of Tunis, and 

 Maritimo on the coast of Sicily ; but owing to the motion of the vessel, as 

 well, probably, to defect in directing the flash truly, from the height to an 

 object not visible, it did not prove successful. 



He considered under such circumstances — that is, seeking for a flash at very 

 great distances beyond the limit of common vision, unless the calculations of 

 the two positions approximated very closely to the truth — that great diflSculty 

 would be experienced, and unless the reflecting plates were very perfect and 

 truly fixed with relation to the directing telescopes, success could not be hoped 

 for at such great distances. 



He had himself witnessed the effects of the heliostadt used in connection 

 with the survey of the country surrounding St. Paul's, and in particular one 

 distant 40 miles : as seen from the station above the cross of St. Paul's, the 

 object was intensely luminous, too much so to be observed with the precision 

 required, as it occupied the whole system of wires. 



