. III. MISCELLANEOUS. 641 



3134;b. Folding Alhidada or Sight- Vane, large size, for 

 topographical surveys. M. Georges Sarasin, Geneva. 



With the plane table to scales of from -^i- to 2 Q - , so constructed as to 

 give relatively with considerable accuracy, notwithstanding its small volume 

 for the sake of portability, measures of distances by the aid of a sight stadia, 

 the measure of inclinations by means of a clinometer furnished with a verneir 

 which gives a reading to three minutes, and consequently the measure of the 

 differences of level, as well as a sketch of the horizontal lines on the ground. 

 It is possible to check any inaccuracies of centering by verniers diametrically 

 opposite to one another, and by turning the telescope end for end. There are 

 also means of regulation and correction. 



3135. Signalling Apparatus (Aeroclinoscope), by Major 

 Kromhaut. -Prof. Buys-Ballot, Utrecht. 



This apparatus consists of four movable discs and two fixed hollow 

 cylinders. Two of the four movable discs never enter the space between the 

 cylinders ; the other pair remain constantly between the fixed cylinders, but 

 may very easily be placed in six different positions clearly to be distinguished 

 from afar. 



The^two discs fi may be brought very easily close to one another and to 

 the first cylinder or the second cylinder ; or they may be more or less 

 separated, one close to one of the cylinders, or both or neither of them. We 

 have then six combinations. Each of the two other movable discs may be 

 hidden, also by another chord, by the cylinder next to it, or be placed close 

 to it or at a double distance. Three positions for each give nine combina- 

 tions (6 x 9 = 54). 



On the whole, when we make use of two pairs of discs we have 54 

 different signals, visible from far in a very distinct way. 



Now, in Holland, the upper outer discs are to represent the barometer- 

 height at Helder, Groningen, and the under outer discs the barometer-height 

 at Flushing and Maestricht, and these form at the same time the direction 

 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. 



3135b. Gauss's Heliotrope, modern construction. 

 Both instruments were constructed in the years 1821 and 1822. 

 Geodetic Institute of the Observatory at Gottingen, Prof. 

 Dr. Schering. 



3135c. Heliotrope, an instrument for throwing the reflected 

 light of the sun in any required direction. 



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



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 

 asphaltum 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. 



39508. S s 



