I. SURVEYING. 727 



2947. Troughton and Simms' 14-inch Theodolite. 



This instrument is one of excellent construction. The telescope has 2 

 inches aperture and 18 inches focal length, the ordinary magnifying power 40. 

 The horizontal circle is read by three micrometer microscopes : the vertical 

 circle by two micrometer microscopes. It is admirably suited for astrono- 

 mical observations. It is, comparatively with the other theodolites exhibited, 

 a new instrument. 



2948. Airy's Zenith Sector. 



This instrument was used between the years 1842 and 1850 in the deter- 

 mination of the latitudes of 27 stations of the principal triangulation of the 

 kingdom. On the destruction of Ramsden's Zenith Sector in the great fire 

 at the Tower of London this instrument was invented and constructed under 

 the direction of the Astronomer Royal, Sir George Airy. The first principle 

 in the instrument is the arrangement for making successive observations in 

 two positions of the instrument, face east and face west, at the same transit 

 of a star. The second principle was the substitution of a level or system of 

 levels for the previously used plumb-line. The third principle was the cast- 

 ing in one piece, as far as practicable, of each of the different parts of the 

 instrument, in order to avoid the great number of screws and fastenings with 

 which most instruments are hampered, and to secure if possible perfect 

 rigidity. 



The focal length of the telescope is 46 inches, the aperture 3*75, and 

 the magnifying power 70. The vertical arc is read by four micrometer 

 microscopes. 



The weight of the entire instrument is rather more than half a ton. 



2949. Zenith Telescope. 



This instrument has done much service in North America and in Scotland 

 in the determination of latitudes. It is most simple in its construction, con- 

 sisting of little more than a good telescope capable of being set to any zenith 

 distance, and rotating round a long and very firm vertical axis. In the focus 

 of the eyepiece, besides five ordinary transit wires, is a micrometer wire, 

 adapted to measurement of zenith distance, to the extent of 45'. The mode of 

 using the instrument is this : suppose two stars pass the meridian at nearly 

 equal zenith distances, and within a few minutes of one another in point of 

 time, the one star being to the north, the other to the south of the zenith ; the 

 observer sets the telescope for the first star (of course in the plane of the 

 meridian), reads the micrometer wire, rotates the instrument 180 round the 

 vertical axis, and looks for the second star : as it passes through the field he 

 bisects it with the micrometer wire. The difference of the reading of the 

 micrometer on the two stars leads to the immediate knowledge of the latitude ; 

 the indications of the level being of course taken into account. This instru- 

 ment was made by Wurdemann of Washington, United States, America, for 

 the North American Boundary Commission : it is a great favourite with 

 observers, and certainly leads to the most excellent results, 



2950. Portable Transit Instrument. 



The small transit instrument exhibited has a telescope of 21 inches focal 

 length and 1 7 inches aperture. The uprights are of mahogany. The telescope 

 is provided with a reversing apparatus, 



2951. Clinometer. Tft 



Eor determining the values of levels, such as are used in the zenith sector, 

 zenith telescope, and other astronomical instruments. The two micrometer 



