II. HYDROGRAPHY. 733 



INSTRUMENTS COMPRISED IN THE ORDINARY OUTFIT OF ONE OF 

 H.M. SHIPS EMPLOYED ON SURVEYING SERVICE. 



2972. Azimuth and Altitude Instrument. 



2973. Theodolite, 5-inch. 



2974. Sextant, observing, and stand. 



2975. Sextant, used for measuring angles between terrestrial 

 objects, to fix the position of a ship or boat when engaged in 

 sounding. 



2976. Sextant, pocket or box. 



2977. Artificial Horizon, roof. 



2978. Artificial Horizon, black glass, portable. Used in 

 travelling. 



2979. Micrometer Telescope. Rochon. Invented by the 

 Abbe Rochon. Date 1812. Used (in surveying) for the mea- 

 surement of distances by means of a base a few feet in length. 



It is formed of two prisms of rock crystal, the one cut parallel to the axis 

 of the crystal, and the other parallel to one of the faces of the pyramid ; these 

 are placed one on the other in contrary directions, and cemented together in 

 a cell which slides in the body of the telescope, and by means of a slit in the 

 direction of its length, so that the prism may be moved along the tube by 

 means of the index, and a milled head screw, which is connected with the 

 prism-box inside. 



The scale on the telescope tube is divided into minutes of arc, and by means 

 of a vernier, reading to seconds. 



A table accompanies the instrument, giving the distances for the angular 

 measurement of each foot of base. 



2980. Heliostat. A mirror for reflecting the sun's rays, 

 used in trigonometrical surveys to indicate, from great distances, 

 the position of a station. 



2981. Station Pointer, 6-inch. For placing the observer's 

 position on the chart from angles taken between three objects, the 

 relative positions of which are known. 



2982. Protractor, circular, with arms. For projecting the 

 angles of a survey. 



2983. Protractor, circular. Bullock. 



2984. Protractor, semi-circular. 



2985. Protractor, rectangular. 



2986. Protractor, horn. 



2987. Parallel Rulers. 



2988. Scale, brass, graduated. 



