III. MISCELLANE 



31071. Davis's Quadrant. 



31O7J. Pocket Sextant, by Gary, used by Captain Henry 

 Bristow, of the Quarter master- 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. 



H. W. Bristow, 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 It has been superseded 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 bore-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. 



31O8. Drawing of a Horizontal Goniometer for deter- 

 mining geographical longitude without a chronometer. 



H. Haedicke, Demmin^ Pomerania. 



The instrument of which this is a drawing serves, in the first place, to 

 determine by direct reading the angle formed by the line joining two stars 

 (a b) with the horizon. The handling of the instrument is for this purpose 

 similar to that of the sextant ; that is to say, the moment must be noted when 

 the star line is covered by the hair line of the instrument. That an observa- 

 tion may be carried out on board ship, the instrument is provided with an 

 arrangement which enables the, position of the scale with respect to the 

 artificial horizon to be fixed at the moment of the observation, so that the 

 angle can afterwards be quietly read by means of a vernier. 



When in this manner the angle of a second star line (c d) to the horizon 

 has been determined, a simple subtraction sum will give the angle between 

 the star lines a b and c d. Should there be a planet amons: the stars 

 a, 6, c, d, it becomes possible, by means of a proper astronomical table, to 

 calculate the astronomical time as well as (if the local time be known) the 

 geographical longitude of the place of observation. 



3109. Tacheometer of Gentilli, a telescopic instru- 

 ment for measuring heights and distances in surveying difficult 

 country. Without calculations it measures accurately distances 

 up to 400 meters, or over 1,300 feet, with an error of less than 

 2-Q-&Q' Dr. Karl von Scherzer. 



This instrument was invented by M. Amadeo Gentilli, an eminent Austrian 

 engineer at Vienna. Its use is for the measurement of heights and distances in 

 the survey of difficult ground, and it has proved especially useful in surveying 

 the contour-lines of mountainous districts. The means by which it measures 

 the distance is an apparatus which obliges the lunette to traverse a precise and 

 unvarying angle. The test of value of this instrument is the fact that, with 

 a magnifying power of 40, it measures distances up to 400 meters with such 

 exactness that the maximum error is less than -- f tne distance. 



31O9a. Laslett's Metroscope. For measuring inaccessible 

 heights and distances, and for levelling ; also used as an optical 

 square. Thomas N. Laslett. 



