II. HYDROGRAPHY. 611 



2987. Parallel Rulers. 



2988. Scale, brass, graduated. 



2989. Scales, ivory ; set of 6. 



2990. Beam Compass, wood. For the accurate measure- 

 ment of long sides in projecting triangulation. 



2991. Drawing Instruments ; magazine set. 



2992. Drawing Instruments, for service in boats. 



INSTRUMENTS OCCASIONALLY FURNISHED TO OFFICERS ENGAGED 

 IN NAVAL SURVEYING EXPEDITIONS. 



2993. Reflecting Circle. Troughton. 



2993a. Allen's Reflecting Circle, for repeating the obser- 

 vations and reading the angle on any portions of the circle, so as 

 to eliminate errors of graduation or workmanship. 



W. Watson and Son, London . 



2994. Sextant, Travelling, small. 



2995. Sextant, Observing, with micrometer and indicators. 

 Davis. 



2996. Sextant, Sounding, Double. Beechey. 



2996a. Rowland's Patent Sextant. Consists of two 

 sextants on the pillar frame principle, mounted parallel, with their 

 faces towards each other, in such a way that one telescope answers 

 for both. One sextant measures angles in a forward direction, 

 the other in a backward, so that if an angle between two points be 

 measured on the larger sextant, and a third point taken between 

 the other two, the angle between this third point and the other 

 two can be measured at one observation, and the sum of these two 

 angles should equal the first. W. Watson and Son, London. 



2997. Sextant, Pocket, Double. George. 



2998. Dip Sector. By Troughton. For measuring the angle 

 between the apparent and true horizons, from an elevation above 

 the surface of the earth. 



2999. Artificial Horizon, portable. George. 



3000. Stadiometer. Blakey. An ordinary telescope fitted 

 with a sliding measure and linear arrangement on object-glass ; 

 designed for the purpose of determining the distance apart of two 

 ships, the height of the masthead of the second ship being known. 



-Qq 2 



