III. SURVEYING. 755 



3O33. Large Double Sextant (6 in. radius) for taking two 

 lunars simultaneously ; an instrument for travellers. Made by H. 

 Porter. Capt. C. George, R.N. 



3O33a. Capt. George's Improved Double box Sextant. 



Made by H. Porter. Capt. C. George, R.N. 



This instrument enables the observer to take right and left angles simul- 

 taneously, thus combining rapidity of use with great portability. 



3033b. Box Sextant with improvement of wheel head to 

 telescope, and case. Made by H. Porter. Capt. C. George, R.N. 



3107a. Improved Box or Pocket Sextant. To enable 

 the observer, when sounding from a moving boat or vessel, to 

 take both angles in fixing a position, without delaying to read off 

 one angle before taking the other. Capt. J. E. Davis, R.N. 



This is effected by means of a supplementary arc and vernier, not affected 

 by moving the proper arm unless connected by means of the hook and pin. 



When used, the arms are connected, and the larger angle is taken first, the 

 hook is then pushed back, and the supplemental arm remains to record that 

 angle, which is read off after the small angle is taken. 



Particularly useful when fixing positions in sounding and the boat or vessel 

 is moving quickly. 



3107b. Improved Sounding Sextant. To enable the ob- 

 server, when sounding from a moving boat or vessel, to take both 

 angles in fixing a position, without delaying to read off one angle 

 before taking the other. Capt. J. E. Davis, R.N. 



When used, the pawl of the indicator is attached to the movable arm and 

 moves with it. When the larger angle has been taken, the finger is applied 

 to the capstan-headed screw, which at once clamps the indicator and frees the 

 pawl from the arm, the smaller angle is then taken and read off ; the arm is 

 then moved up to the indicator and the larger angle read off ; the capstan- 

 headed screw is then loosened and the pawl drops, connecting the indicator 

 again with the arm. 



3107c. Improved Sextant. For observing and recording a 

 number of observations without the necessity of reading off at 

 the time of observation, or removing the eye from the telescope, 

 effected by means of a micrometer movement affixed to the tan- 

 gent screw, and indicators applied to the arc. 



Capt. J. E. Davis, R.N. 



It is particularly adapted for observing lunar distances, circum-meridian 

 altitudes, equal altitudes for time. For position in bad weather, &c. 

 Its advantages are: 



1. It enables the inexperienced observer to take observations with as 



much facility as-the more practised. 



2. By greatly multiplying the number of observations, the instrumental 



and personal errors are reduced to a minimum. 



3. At sea, it enables a number of observations to be made in a short 



time without being dependent (as is often the case) on one. 



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