2 STATION POINTER FeR MARITIME SURVJYi, 



•* r '^^"^^ It confifts of a circle divided into 360" and filled with three 



Delcription« 



It is a piotraaor radii, C B, C D and C A ; the firft of which is fixed at zero 

 witi three radii, ^^^^ ^^ ^jj^^^. ^^^ are moveable, and have each a nonius fhew- 

 vrhich being fee ^ . r«, ^ • r j 



mnd duly placed ing fingle minutes of a degree. The centre at C is pertorated 

 on a chart givei j^^ ^^^^^ jq admit the pointer L, which is cylindrical and well 

 the ftationofthc , , » , -^ r . • r j * 



©kfcrver, fitted to the hole, where it ferves to give a correlpondent 



mark upon the chart or paper when in ufe. I is a magnifier 

 carried by an arm, having its refledor K to throw light on the 

 divifions, and its cylindrical ftem H to be inferted in the hollow 

 centre at C. As it muft, while in this fituation, prevent the 

 introduction of the fteel point L, the ftem H is itfelf perfo- 

 rated in order to allow a fmaller fteel point M to be introduced 

 for the fame purpofe. The radii A B and D are of brafs 5 but 

 they may be prolonged to an extent that would render them 

 heavy and inconvenient if of metal, by the addition of three 

 rulers of wood, which are attached when required by milled 

 head fcrews and fteady pins, as fliewn by breaking the radius 

 A, and alfo in the fide view at G. 



The arms D and B are capable of being brought very near 

 each other, and confequently of being fet to a very fmall an- 

 gle; but A cannot, on account of the noniufes, be brought 

 within ten degrees of D. Whenever, therefore, the left hand 

 angle happens to be fmall, it becomes neceflary to carry the 

 arm A round, until by its fituation on the other fide of B, it 

 comes to be employed to take the right hand angle, and con- 

 fequently leaves the left to be taken between D and B. 



The pointer A is provided with a clamp, by means of which, 

 that arm can be firft fecured in its place, and the pointer D is 

 then fet without fear of difturbingthe fituation of the arm firft 

 fet. It feldom happens that both angles are fmall in maritime 

 furveying; fo that the condition that A cannot be brought very 

 near either of the other arms, has not been found produdive 

 of inconvenience. 



In Fig, 1 . Flait III. let A, G, and M reprefent three objects, 

 of which the actual diftances from each other are known and 

 projected in a chart. Suppofe an obferver at H to have mea- 

 fured the angle A H M=22®.40', and the angle M H G = 

 53®. 59' it is required to protradt the place H by means of the 

 ilation pointer. 



Solution. Set the arms of the inftrument to the refpeftive 

 angles, and apply them to the chart fo that each point A, M, 



