in. -SURVEYING. 767 



of the expected wind. They show the azimuth of the gradient (the strike). 

 The two inner discs denote the magnitude of the gradient (the fall). 

 All these positions are to be seen in the joined diagrams. 



3135a. Gauss's Heliotrope, ancient construction. 



Geodetic Institute of the Observatory, Gottingcn, Prof. 

 Dr. Schering, Director. 



3135b. Gauss's Heliotrope, modern construction. 

 Both instruments were constructed in the years 1821 and 1822. 

 Geodetic Institute of the Observatory, Gottingen, Prof* 

 Dr. Schering, Director. 



3135c. Heliotrope, an instrument for throwing the reflected 

 light of the sun in any required direction. 



Prof. W. H. Miller, M.A,, F.R.8. 



The Heliotrope consists of a plane glass mirror, two adjacent edges of 

 which are ground and polished in planes making right angles with one 

 another and with the large planes of the mirror, and then covered with 

 asphalte varnish.. A portion of the silvering, not larger than the pupil of 

 the observer's eye, is removed from the angle where the two small polished 

 surfaces meet the hinder plane of the mirror. 



If held in such a position that the sun's light falls in the solid angle 

 between the face of the mirror and the two small polished surfaces, a portion 

 of the sun's light that falls upon the face of the mirror is refracted at the 

 first surface, reflected internally at each of the small surfaces, and finally 

 emerges through the space from which the silver has been removed, in a 

 direction parallel to, but opposite to, that in which the reflected light travels 

 from the large plane of the mirror. 



Hence, any point with which the faint image of the sun appears to 

 coincide will receive the light of the sun reflected from the mirror. 



3136. Sun Signals, for the use of travellers. 



Francis Gallon, F.R.S* 



The difficulty in sun-signalling is to direct the flash aright. The rays of 

 the sun are reflected from a mirror, in a cone of light precisely similar to that 

 which reaches it, the mirror itself (whose size may be disregarded) being the 

 apex of the latter cone, and the sun's disc its base. It follows, that to the 

 signaller, whose eye is near the mirror, the place where the cone of reflected 

 rays falls on the distant landscape would always appear to him as a disc of 

 precisely the same shape and size as the sun itself. In other words, his 

 accuracy of aim must be within 30 minutes of a degree. In the author's 

 heliostat an image of the sun is produced, which overlies the area on which 

 the flash of the mirror falls. A lens is fixed in the instrument at right angles 

 to the line of sight ; half of the lens lies within the tube through which the 

 observer looks, and occupies a portion of his field of view, the other half is , 

 external to his field of view ; it projects beyond the side of the eye tube, and 

 receives the flash of the mirror. The mirror turns on an axis attached to the 

 tube, which allows it movement in one direction, while the rotation of the 

 entire instrument in the hand gives movement in the other. When the mirror 

 is so adjusted that the reflected (parallel) rays from any one point of the 

 ^un's disc impinge on the lens, they are brought to a focus on the screen, and 

 form a minute speck of light upon it. Kays radiate from this speck in all 



