c20 



Reflecting Semicircle in Military Sketching. 



[Oct. 



on the scale assumed. But in making an extended and accurate sketch 

 of a country, it is necessary to find the situation of a number of stations 

 all over it, from which the rest is drawn in, at least once in 

 each square mile, if an accurate sketch of the detail is required 

 or any thing more than the general features are intended to 

 he laid down. The operation technically termed, finding the 

 station, is that in which the situation on the draft of any particular 

 place, is found by the angles observed as subtended from it by 

 the known points around. For this purpose the instrument may 

 be thus applied. Let 1 2 3 4, fig. II. represent the folding field sketch. 



Draw the line A B, to represent the " 



magnetic meridian at the place. 

 Let C D F G, be points which have * 

 been laid down trigonometrically 

 and letF be the station required to 

 be found. With a surveying com- !*'-'-''" 

 pass take the bearing of one of the ! 

 points from F as of C, and draw the 

 occult line H I, making the angle j \ 

 HI A, equal to the bearing of C 

 from the meridian, (to protract this ; ^ - - -M 



angle the circle will be convenient), 

 and draw C F passing through C 

 parallel to HI. Then it is plain ; 

 that the required station F must hiy \ 

 somewhere in the line F C. With b4 y 



the circle observe the angle sub- 1 

 tended from F between C and D, i 



C and G, &c. and protract these an- L 



gles by laying the instrument on the * 



sketch with one of its arms parallel to the line FC, and move it until 

 the line of the other arm intersects the points D and G, the angles 

 protracted CFG, and C F D, will then be equal to the observed 

 angles subtended by those points, and the place of the station as re- 

 quired will be at F. If the sketch is on the scale of one inch to a 

 mile, the arms of the instrument will not be long enough to reach 

 from every station as protracted to the points, but the line of the arm 

 can be seen very nearly with the eye, and the intersection of several 

 angles will ensure accuracy, or a ruler or a horse-chain may be used 

 as an assistance. 



These operations may all be completed in the most rapid manner 

 without getting off the horse, and the method combines both accuracy 

 and quickness. 



