"ART OF TRAVEL" 165 



much of what I had wished should be known by 

 them. 



A small contrivance of my own, over which I 

 spent a great deal of time, may be alluded to here ; 

 it is described at length in the Art of Travel, and in 

 other publications, as a " Hand Heliostat " [10]. I 

 contrived and practised with it long before the present 

 system of sun-signalling had been invented. The use 

 of a heliostat for creating a point of light, visible at 

 great distances for purposes of Ordnance triangulation, 

 had long been fully recognised ; a description of its 

 employment from Snowdon to Scawfell has already 

 been given in Chapter V. The difficulty in using a 

 portable instrument is to direct the flash with sufficient 

 accuracy of aim. If the part of the landscape upon 

 which the flash falls could actually be seen by the 

 operator, it would always appear to be of exactly the 

 same size as the disc of the sun itself, whatever the 

 distance may be ; in other words, it subtends an angle 

 of about 30 minutes of a degree. My plan was to 

 divert a small part of the flash so as to create a mock- 

 sun in the field of view of the instrument, which the 

 operator could throw, by judicious handling, upon any 

 desired spot in the landscape, with the assurance that 

 persons on the ground covered by the mock-sun could 

 see the flash. The instrument is now used in nautical 

 surveys, as I was told by the late Hydrographer, Sir 

 William Wharton, to enable shore parties to make 

 their exact whereabouts visible to those on the ship. 

 The heliostat that I usually carried with me went 

 easily into a large waistcoat pocket, and was very 

 efficient at a distance of ten miles. I should have 

 been glad to possess one on many occasions when 



