III. MISCELLANEOUS. 629 



3090a. Surveying Table, with Plummet Rule, as used 

 by the topographical surveys in Russia. 



The Topographical Department of the Imperial Russian 

 General Staff at St. Petersburg. 



The plummet rule is provided with an altitude circle, and, besides, ar- 

 ranged as an instrument for measuring distances by adjusting the horizontal 

 strings of the two water-levels which are attached to the plummet rule ; the 

 lower one is for regulating the horizontal position of the surveying table, 

 the upper one for regulating the horizontal position of the altitude alhidada, 

 by means of a micrometer screw acting against the beam of the alhidada. 



3091. Jaehn's Polymeter, with plane-table. 



Schmidt and Hacnsch, Berlin. 



Jaehn's " polymeter" enables the measurement of the distance of two points, 

 or their relative position in the horizontal direction, and their differences of 

 altitude to be obtained in a mode quite different from previous methods of 

 operation. Both may be effected 1, simultaneously ; 2, by one observation ; 

 3, from a single point ; 4, without any calculation ; 5, by a mechanical setting 

 out of the measured distances and heights in proportion upon the plane of 

 the measuring table ; 6, in any scale. 



3092. Double Prism, on Jesse's System. 



Schmidt and ffaensch, Berlin. 



3093. Experimenting Model (quarter of the natural 'size) 

 for the preparation of measuring bars to a new base apparatus. 



Carl Bamberg, Berlin. 



Experimental Model. The steel tube is bored through cylindrically in 

 an axial direction, closed at the ends and filled with mercury, which also 

 passes into a glass tube, closed above, and communicating with the boring. 

 The height of the column of mercury in the glass tube will be a measure of 

 the alteration of the length of the scale by change of temperature. The pro- 

 portions of the cavity of the passage in the steel and glass tubes are so 

 arranged that every change of length in the scale is increased 100 times in 

 the glass tube. 



This instrument has been made as an experiment by order of the Royal 

 Prussian Laud Survey. 



3093a. Drawing of a Base Measuring Apparatus, with 

 employment of the measuring bars mentioned with the above appa- 

 ratus. Carl Bomber g, Berlin. 



3094. Distance and Altitude Measuring Plumb-line 



(telescope ruler), executed, according to the directions of Prof. 

 Helmert, by J. W. Breithaupt and Son, Cassel. 



Prof. Helmert , Aix-la-Chapclle. 



The telescope ruler is also for topographical surveys. It has a distance- 

 measure on glass ; the altitude-circle is furnished with two verniers, although, 

 as a rule, only one of them need be read off; moreover, the position of its 

 index may be ascertained with the lens. A spirit level is combined Avith the 

 vernier arm ; a mirror enables it to be seen from the eye-piece. The instru- 

 ment also has levels and adjusting screws for the horizontal axis of rotation 

 of the telescope, and also parallel ruler. 



