HELIOGRAPHY (sun telegraphy), a name given to a method of communicating between distant stations by reflections of the sun upon a mirror or system of mirrors. One great advantage of this method over the ordinary signal system is that the appa ratus is more portable, but it can be successfully used only in regions where the atmosphere is clear of clouds through considerable periods of time. Anybody who has any idea of the ordinary method of telegraphing by electricity with the ear alphabet, will understand how spaces of time may be employed to indicate letters and words by means of the eye. There are two methods: the reflector may be obscured except when the screen is temporarily removed to produce a flash or letter; or the reflector may be kept exposed except when it is obscured to produce a letter. The first method is said to be the easier for the beginner, but the second less fatiguing to the eye. The distance
through which this mode of communication may be carried on varies with the size of the mirrors and the clearness of the atmosphere. From the Himalayas a 5 in. mirror has communicated distinct signals 60 miles. The instrument could be used with good success on the Andes. When the signaling station forms an angle greater than a right angle between the sun and the receiving station, two mirrors are used to prevent too great a loss of rays by oblique reflection. The mirrors are mounted on tripods, and are held by a socket, or a universal joint. Besides its use as a signaling instrument, the heliograph has served to define distant points in a survey, and for this purpose was. employed in the triangulation of India. It was also used by the late astronomer-royal of England at the cape of Good Hope in verifying the arc of the meridian.