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"ART OF TRAVEL "   165

much of what I had wished should be known by

them.   I

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 " [io]. 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 3o minutes of a degree. My plan was to divert a small part of the flash so as to create a mocksun 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