750 SEC. 15. GEOGRAPHY. 



which mirror and prism are fixed, means are given for cross observation to 

 the extent of 288, &c. Also, the stand for observations on shore is in parts 

 changed and endered more practical. 



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. 



1722b. Double Reflecting Circle, by Capt. Owen, R.E. 

 The property of the Rcyal Astronomical Society. 



Robert J. Lecky, F.R.A.S. 



This invention of the late Capt. Owen, R.E., is peculiarly valuable for the 

 readiness with M r hich two observations or " sights " can be taken with it, 

 without removing the instrument from the eye, e.g,, in a lunar observation 

 at sea the altitude of the star and the lunar distance may be taken with great 

 rapidity, as well as in other observations. 



The two circles each six inches diameter are divided on silver and read by 

 three verniers on each to 20 seconds of arc, and the glasses being placed 

 between the circles are well defended from accident at sea. It has other 

 adaptations of much ingenuity. 



3038a. Repeating Circle, of 1 foot diameter, by Troughton. 



Royal Society. 

 3111. Portable Repeating Circle (14 centimetres). 



J. $ A. Molteni, Paris. 



3088a. Prismatic Compass (german silver). 



G. W. Straw son. 

 3088b. 3 .\-inch Prismatic Compass (bronze). 



G. W. Strawson. 

 3088c. Singer's Gilt Compass. G. W. Strawson. 



c. LEVELS. 



3037. Large Levelling Instrument by Gary. 



Royal Society. 



3038. Large Levelling Instrument by Troughton and 

 Simms. Royal Society. 



3O63. Levelling Instrument with telescope of 15" focal 

 distance, I 1 6" aperture; a so-called compensation plane with 

 rotating level. Ott and Coradi, Kempten^ Baviera. 



This " compensation level " is a levelling instrument with telescope, which 

 can be turned round its optical axis and reversed in its sockets. The water 

 level is firmly fixed to the telescope, and can be turned in sockets rourfd its 

 own axis in such a manner that on rotating the telescope through 180 the 

 scale of the water-level can be always turned upwards. 



The object of this arrangement is to eliminate the errors occurring by 

 damage of the telescope rings, and to give to the engineer at all times the 

 possibility of correcting the instrument as easily from any position as in a 

 levelling instrument with reversible telescope, with exactly equal ring dia- 

 meters and adjustable water-level. By this arrangement the tedious reversing 



