756 SEC. 15. GEOGRAPHY. 



4. The differences of the altitudes being all equal, the check on the 



time-taker is apparent. 



5. For night observations it is peculiarly adapted. 



6. The micrometer movement can be thrown out of gear at pleasure. 

 The sextant has been submitted to the Astronomer Royal, who states that 



" the arrangement is simple, very little liable to get out of order, and I should 

 " think very effective." 



The many sources of error to which the astronomical sextant is liable, 

 either from the different expansive qualities of the materials of which it is 

 composed or the mechanical difficulties attending its construction, render it a 

 less perfect instrument than it is generally supposed to be. 



There is also a personal error, incidental to every observer with the sextant 

 or any other instrument, and this is frequently augmented in night observations 

 by the necessity of dilating the pupil to the utmost when observing, and 

 suddenly contracting it in reading off by the aid of the strong rays of a bulFs- 

 eye lantern. 



With these sources of error, any improvement that can be devised to bring 

 either the instrument to a greater state of perfection, or to deduce a greater 

 degree of accuracy from the observations taken with it, is a step in the right 

 direction, and it is anticipated that by the use of the micrometer movement 

 combined with the indicator, the number of observations can be so multiplied 

 as to reduce the error to a minimum. 



3107h. Sextant by Syeds, to be used in a fog. 



Capt. C. George, R.N. 



The use and design of this instrument is to give a correct altitude when 

 the horizon is obscured by fog, &c., which desirable end is obtained by this 

 quadrant the addition of a projecting bar, containing a glass level and the 

 bubble floating therein, which, when seen in the horizon vane in connexion 

 with the sun, gives the true altitude. 



31O7d. Double Telescope Sextant, with reduced arc and 

 natural angles. Patrick Adie. 



This instrument was invented and patented by Mr. Adie some years ago. 

 but owing to difficulties in simplifying the adjustments, not yet surmounted 

 for lack of time, has not been publicly introduced. It is an arrangement of 

 the object ends of two telescopes, united by a common hollow axis ; the rays 

 from each are received on total or other reflecting surfaces placed at an 

 angle of 45 at the centre of either end of this axis, along which each of the 

 reflectors sends the rays at right angles ; these rays are received by two 

 other reflecting surfaces, each occupying half the field of the oye-piece. Thus 

 both objects are seen at the same time. A motion half that of the mov- 

 able telescope is given to the eye-piece, to correct the parallax of reflection 



from prisms placed at an angle of 45. - 



3107e. Cary's Improved Edge Bar Sextant. Special 

 construction to prevent expansion or contraction in different 

 temperatures. Henry Porter. 



3107f. Portable Sextant. John Browning. 



3107g. Six-inch Sextant. John Browning. 



31O7L Davis's Quadrant. H. Porter. 



