CHAP, v.] USE OF THE SHIP FOR TRIANGULATION 151 



bisecting the angle G C D, and at such a distance from C as to 

 make the receiving angles E H C, G D H not less than 30° or 

 40°, taking care to keep within easy visual range of E. The 

 angle G D H is roughly known by the difference in the bearings 

 of D from G and H. 



It will materially assist the ship in picking up her proper 

 positions, if she first steams along the coast and cuts the marks 

 in by means of a running survey by patent log and compass. 

 Having laid down the floating beacon E, a base may be 

 measured either by sound or range-finder between E and B, or 

 by chaining and traversing between A and B ; or in the case of 

 an extended survey by this method, the scale may be eventually 

 determined from the astronomical observations at either end of 

 the survey, in which case it is not necessary to measure a base. 



Observers with theodolites and heliostats being landed at 

 A, B, C, and D, the ship takes up her positions successively 

 at F, G, and H, At preconcerted signals, the ship is shot up 

 simultaneously by each observer at each successive position, 

 the angle to the beacon being also noted on each occasion. 

 The observers in the ship at F take the angles E F B, B F C, 

 and at G the angles B G D, B G C, E G B are observed in the 

 order named, for reasons that are stated on p. 108 ; at H, the 

 angles BHD, CHD, EHB aie also observed in the order 

 named. 



The ship has thus never more than three angles to observe at 

 each position, and they should be taken as quickly as possible 

 to avoid any error creeping in through a slight movement of 

 the ship between the observations. Two observers will 

 materially diminish any risk of this sort. 



The object of observing the floating beacon on each occasion 

 from the shore stations, is to detect any slight movement from 

 the position it occupied at the moment of observing at F. 

 The theodolite lines to E, from A and B, effectively control 

 its position. From the difference of the angles to it at those 

 stations at the instant of observing F and G respectively, the 

 false station which E occupies when the angles at F and G are 

 observed can be calculated on the principles stated on p. 92, 

 and a correction for it can be applied to the angles E F B, 

 E G B as actually observed, to reduce those angles to what 

 they would have been if the beacon had not moved. A cor- 



