CHAP. III.] BASE BY SOUND 73 



yards consistently with a percentage of error equal to that of 

 the 9 feet range-finder at 2 miles. 



MEASURED BASE BY ROPE. 



Measuring by a rope is of course not accurate. It is difficult 

 to avoid stretcliing it more at one time than at another, and 

 if it gets wet, it alters its length considerably. If measuring 

 over ground where it is sure to get wet, it will be better to wet 

 it well beforehand. Test it in that condition, and keep it well 

 wet all the time of measuring. 



BASE BY SOUND. 



This consists in counting the interval of time which elapses 

 between the sight of a flash of the gun and the arrival of the 

 sound of the explosion, the gmi being at one end of the required 

 base, and the observer at the other. 



Recourse is had to this method of obtaining a base when no Final 



flat ground can be found on which to measure. Its accuracy is ^/^*^® 



"^ *^ should not 



not great by any means, but, if the final scale of the chart is depend on 

 to depend on astronomical positions, it is quite near enough f*^® ?^ 

 for working out details such as heights, small parts measured 

 with 10-foot pole, etc. 



Its value is much increased by observing from both ends, Useful 

 which should always be done if possible, and a surveying vessel ^^"*^ 

 should have two small brass Cohorn mortars supplied for this 

 purpose, which can be sent away in a boat, and tumbled over- 

 board without damage. 



The ship is often used at one end of a base by sound, 

 especially in work amongst small islands, and it is also neces- 

 sary sometimes to have a boat at the other, but if at any rate 

 one shore station can be obtained it will be better. If choice 

 of direction can be had, measure with the wind across the 

 base, as, though the error from increase and decrease of 

 velocity is eliminated by measurement from both ends, 

 the sound may be difficult to hear against the breeze, if 

 at all strong. 



For either end choose positions for the hearers as much out 



