CHAP. TIL] BASE BY SOUND 77 



is " shot up " by the observer at the theodohte ^, and the 

 observer in the fore-top at the same instant measures the angle 

 between the base A and the conspicuous object. This object 

 thus practically becomes the other end of the base, the ship 

 being eliminated altogether ; a separate value is deduced for 

 the shore base, corresponding to each gun and each different 

 position of the ship at the moment of firing. If the wind is 

 blowing across the line joining the theodolite A and the ship, 

 it will not be necessary to land a mortar for return signals ; but 

 it is preferable to do so. The formula 



T= 



is of course applicable to this case. 



The following details for carrying out the above may be Details 

 useful : 



1. A single gun from the ship to indicate " Prepare." Flag 

 hoisted simultaneously at the fore, at the dip. 



2. Flag will be mast-headed one minute ten seconds before 

 firing each gun. 



3. Flag will be dipped ten seconds before firing. 



4. Observe the flag by the theodolite as it is dipped on each 

 occasion ; note the interval between flash and report, and read 

 off theodolite directly afterwards. 



5. Flag will be mast-headed as each gun is fired. 



6. At the seventh gun flag will be hauled down. 



7. If the shore observer wishes ship to repeat, he will fire 

 the mortar one minute after ship's flag is hauled down. 



8. Two minutes after hauling down flag it will be rehoisted at 

 the dip. 



9. Flag will be mast-headed, and kept up one minute. 



10. Flag dipped. The mortar is fired from the shore ten 

 seconds afterwards. 



11. Flag is rehoisted on mortar being fired. 



12. At the seventh discharge of the mortar the flag will be 

 hauled down. 



13. If the ship wishes shore to repeat, she will fire a gun and 

 hoist flag at the dip, and the operation mil then be repeated 

 as before. 



