Bauhaus-Universität Weimar

“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 ol Ordnance triangulation, 
had long been fully recognised ; a description of its 
employment from Snowdon to Scawfcll has already 
been given in Chapter V. The difficulty in using a 
portable instrument is to direct the Hash 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 Hyclrographer, 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
        

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