CHAP. IV.] MAKING A MAIN STATION 87 



the event of the zero presently being discovered to be wrong, 

 how far back the angles must be recomrtienced. A common 

 form of notation of this is, Z. 0. K., or Z. K., for zero correct. 



If the zero is found continually getting displaced without Defects of 

 any apparent cause, something is loose, and this must be ^ent!^ 

 looked to at once, or nothing \vill be satisfactory. The parts 

 most liable to go wrong have been mentioned under the head 

 of "Theodolite." 



Placing a heavy stone against each leg of a theodolite, or Precau- 

 one heavy stone triced close up under the apex of the tripod, sTrong^ ^ 

 is very effective in steadying the instrument. The shelter of wind. 

 an umbrella will often enable angles to be accurately obtained 

 which would otherwise be doubtful. 



If using a heliostat, it must be placed in front of the theo- Arrange- 

 dolite, in the direction of the station to which you mean to ^g^° °^ 

 flash. When the stations are distant one from the other, it is Heliostat. 

 desirable to arrange who shall flash first ; the receiver of 

 the flash, say at A from B, then takes his angles to it, and 

 does not direct his flash to B until he has got the requisite 

 number of repetitions. When he does flash to B, the latter 

 will know A has done with liim, and can direct his flash to 

 some other station, while he observes A. When B in turn 

 has finished ^vith A, he must give the latter another flash to 

 acquaint him with the fact. A's turning off Ms flash will 

 show B he understands. 



As already remarked, the amount of time saved when the Heliostat 

 sun is visible, by the use of a heHostat, is incalculable. It i^^aiu- 

 is useful for long distances, and short also, and on all sorts of 

 occasions, and is, in fact, one of the surveyor's greatest friends. 



FALSE STATION. 



It will often happen that a beacon having been erected, the 

 theodolite cannot be placed exactly on the spot, at any rate 

 without a great deal of trouble ; or if a building or tree has 

 been selected as a station, that the observer finds on going 

 there that he has to make his station on one side of it in order 

 to see what he wants, or has to make a supplementary station 

 to see a few objects obscured by the building, etc. This is 

 called " False Station," and if the object is already plotted, or 



