CHAP. I.] TESTING STATION POINTER 31 



Now do the same for the small angle and its " points," and 

 the intersection of the two small arcs will give the position 

 required. 



It is, in fact, projecting the circles by means of the station 

 pointer. 



Station pointers are made with brass, and with silver arcs ; 

 the latter are of course more durable, but for many purposes 

 the brass are to be preferred. When sounding, or doing any 

 work in the open, the reflection from a silver arc is often a 

 bother, and hinders speedy setting of the vernier. The use of 

 a reading glass is almost a necessity with silver arcs for this 

 reason, and also on account of the fineness of the cutting ; 

 whereas, with the brass arcs, a surveyor with good eyes can set 

 his instrument quite correctly without one, a great point in a 

 boat. 



For chart-room use the silver arc is to be preferred. 



The nick in the centre of the instrument should be small. 

 i.e. just deep and wide enough to admit of a needle fairly 

 catching in it. The needle-pricker should always be used for 

 marking the position ; not a pencil-point, which soon wears 

 blunt, and will not mark truly in the centre. The prick also 

 remains, and can be seen under the figure with a reading glass, 

 when inked in. 



The prick should be on the continuation of the edge of the 

 bar in which is the nick. 



For ordinary soundings and field work, a station pointer of 

 about 5 inches diameter of arc is most convenient. For ship 

 sounding and chart-room work larger ones are supplied. 



In testing a station pointer, the first thing is to see that the Testing a 

 vernier of the leg which comes back to zero reads exactly 0°, Pointer 

 using a magnifying-glass to read off accurately. If it does not, 

 the screws which hold the vernier must be loosened slightly, 

 and the vernier plate moved, until the arrow on the vernier 

 corresponds exactly with 0° of the arc, and the 30' on the 

 vernier with a division of the arc. 



Take either a large sheet of backed paper, or a white Bristol Testing 

 board, and mark out, by means of chords, lines radiating from 

 a centre, and 10° apart. These lines must be very carefully 

 ruled, and in Indian ink, as this sheet must be kept as the test 

 of all station pointers and protractors, which should be from 



