III. SURVEYING. 757 



3107J. Pocket Sextant, by Gary, used by Captain Henry 

 Bristow, of the Quartermaster- General's Department, in making 

 the military maps of the north of Spain, for the use of the British 

 army, under the Duke of Wellington, during the Peninsular war. 



//. W. Bristoiv, F.R.S. 



The sextant is a convenient instrument used for measuring the actual angle 

 between any two well-defined objects, in whatever direction they may be 

 placed, so that the angle does not exceed 140 ; and without requiring more 

 steadiness than is necessary for seeing the objects distinctly. 



The pocket sextant was formerly used in military and maritime surveying, 

 for fixing points and for filling in the details of maps. It has been superseded 

 by the form known as the box-sextant (the principle of which is the same as 

 in the larger sextant), which will measure the angle between any two objects 

 to a single minute. 



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. 



e. MISCELLANEOUS. 



3030. Portable Artificial Horizon. Made by H. Porter. 



Capt. C. George, R.N. 



Its improvement consists, not only in its reduced size and weight, but in its 

 mechanical arrangements, form, and moderate price. 



It secures altitudes near the horizon, as low as 3 ; saves time, and prevents 

 waste of mercury. 



Ibs. oz. in. in. 



Large size, weighs in the case - 4 10 measures 9 by 5 

 Small - 1 11 measures 6 by 3 



3035. Inclined Reflecting Horizon, an instrument for 

 travellers. Made by H. Porter. Capt. C. George, jR.N. 



This instrument is used with the usual artificial horizon, to which it may 

 be said to form an appendage or adjunct. By its aid such increased power is 

 given to the sextant and artificial horizon that altitudes of the heavenly 

 bodies can be measured from the zenith to 30 below it, and also altitudes 

 from the horizon to 30 above it and 30 below it ; this the sextant and 

 artificial horizon have hitherto failed to do. It consists of a glass reflector, 

 supported by a framework, which has its underside ground mathematically 

 level, and this side floats on the surface of the mercury, carrying the reflector 

 at an angle of about 30 with the natural horizon, and when properly made 

 will always float on the mercury at the same angle ; it has been tested by 

 numerous observations, with satisfactory results. See observations and 

 drawings. 



