760 SEC. 15. GEOGRAPHY. 



scale at distances of 90, for forming right angles. The distance of the two 

 sights is 30 cm. ; one bears two horizontal cords, in the other a little 

 slide with movable sight, which can be moved in a vertical direction. If 

 the index of this slide stands at zero on the division fastened to the sights, 

 the line of sight is horizontal when the tablet of the instrument is put in 

 action. By moving the slide, and measuring the way which has been passed, 

 the instrument can be used for indirect levelling distance and mensuration of 

 heights and tracing. A chord drawn over both sights allows the formation 

 of perpendicular lines. 



309Oa. Surveying Table, with plummet rule as used by 

 the topographical surveys in Russia. 



The Topographical Department of the Imperial Russian 

 General Staff, 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, 



309 Ob. A. C. Bagot's Patent Clinometer. A. Apps. 



The object of this instrument is to enable those persons who are engaged 

 in engineering, contractor's work, land agencies, mines, &c. to get sufficient 

 preliminary data with respect to the configuration, &c. of the land as shall 

 warrant a more complete survey later, without having to carry about a theo- 

 dolite, or Y level, with their various cumbrous appendages. 



Foremen platelayers can accurately get the gradients of the railway ; the 

 arrow on the brass scale on the right indicates the angle subtended; the 

 arrow on the left simultaneously indicates the number of inches rise or fall 

 per yard. 



Thus, if the limbs are opened until the right-hand arrow marks 45, on 

 referring to the left-hand arrow the number 36 is found ; this means a rise of 

 3 6 inches per yard, or 1 in 1. 



The telescope is reversible and of short focus. Such tables as are fre- 

 quently necessary when in the field are calculated out, and engraved on the 

 side of the instrument for reference. 



The sight of the prismatic compass enables the operator to read off number 

 of degrees, and still keep his eye on the object. 



As a draining level for estate work this instrument will be found unrivalled, 

 since an approximate idea of the amount of fall can be got before going to 

 the expense of having the land surveyed, and being told that the fall is 

 inadequate to the purpose. 



Mining engineers can solve many problems with this instrument where the 

 Y level could not be set up. 



The design and construction, &c. of the instrument is strictly protected by 

 Royal Letters Patent, granted to Alan Charles Bagot, Churchdale, Kugeley. 



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



Schmidt and Haensch, 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 ; 



