4 STATION POINTER. FOR MARITIME SURVEY^,' 



Defcrlptlon and Angle obfervcd between St. Giles's and St. Martin's froiW 



ufcQ/thcftation soho Square 22^. 40'i. 



pointer. ' 2, 



Angle obferved between St. Martin's ant! St. Anne's from 

 ditto 53"^. 59'. 



Conftrudion. Drav<' the parallel of St. Paful's, and fet off 

 the points A, Gy and M by thfe refpedtive drfferences of latitude 

 and of meridians toreprefent the churches of St. Anne, St. Giles 

 and St. Martin. Join A and G, the two objefls moft remote 

 rn their bearings, and from the extremities A and G on the fide 

 of the line fartheft from H, draw the lines Ah, Gl^ making 

 angles with A G rcfpeiStlvely, equal to the angles obferved on 

 coi'itrarj tides of the line pointing to the middle obje6l M. 

 Through A,h and G defcribe a circle; and through M and h 

 draw a right line, which prolonged will cut the circle inH. 

 Join A H, G H, and the angles h A G, M H G, and A G h, 

 MH A will be refpeftively equal, becaufe landing on the 

 feme arcs h G and h A; that is to fay, the objects will be feen 

 from H under the obferved angles, and confequently H will be 

 the place of the houfe. 



Computation. The nutnbers in the figure Were had by care- 

 ful confirudion on a fcale of one inch to equal 100 feet, ufin^ / 

 a beam contpafi;, which divides the inch into 1000 parts. But 

 as the computation may liot be unacceptable to beginners in 

 trigonometry, I will here give the procefs as a conclufion to 

 the prefent paper. 



1. The triangle MAG is known. We have therefore the 

 fide A G and two angles given rn the triangle A h G, which 

 is thus determined, 



2. In the knowii triangle M A G by fubduding the angle 

 A G h from the angle A G M we gain the angle h G M whicii, 

 with h G, G M, determines the triangle h G M. 



3. The angle H h G is (by Euclid I. 32) equal to the 

 fura of the interior oppofite angles h G M, G Mh. There- 

 fore we have the fide h G (by paragraph 1) and the two an- 

 gles H and h in the triangle- H h G. Confequently that tri- 

 angle is known, and the diftance from H, the houfe, to the ob- 

 ject G may be had. 



4. In the triangle M G H the angle at G is equal to the fum 

 of the known angles M G h, h G H and the other angle at H 

 and alfo the fide M G are known. Whence the triangle is de- 

 termined, and the diftance from H to the object M may be had. 



5. In 



