76 HYDROGRAPHICAL SURVEYING [chap. m. 



At B have been observed 



85 beats with watch beating 9 beats to 2 seconds 

 87 



88 



Mean 86- G6 beats = 19-238 seconds. 



47 beats with watch beating 5 beats to 2 seconds 



47 

 48 



Mean 47-33 beats = 18-932 seconds. 

 Mean at B= 19-085 seconds. 



2 tt^ 

 Then working T = 



we get T= 18-500 seconds. 



Temperature is 80°, at which velocity of sound is 1145-2 feet 

 per second. 



This, multiphed into the interval, gives 21,197 feet for the 

 length of our base. 



The temperature must be taken in the open with the thermo- 

 meter shaded from the direct rays of the sun, but not in too cool 

 a spot, or it will not give the true temperature of the free air. 

 Ship Guns fired from the ship, either under weigh or at anchor, 



Two^BTse placed about midway and exactly in a line between the two 

 stations, base stations, give a long base ; provided the wind is blowing 

 right across the line of the base, satisfactory results may be 

 obtained. The base for the survey of the western portion of the 

 Straits of Magellan was obtained in this manner, no other 

 method being practicable. 

 Ship and Another very useful method of using the ship to obtain a 



Single hase by sound, in connection with a single theodohte station on 



Theodo- j > b 



lite shore, may here be mentioned. The ship, not necessarily at 



station, anchor, is placed at about three miles from the theodolite 



station, and in the most favourable position that circumstances 



will allow for giving a good cut to a conspicuous object on shore, 



which is also visible from the theodolite A , and at a considerable 



distance from it ; the receiving angle at that object, between 



the line from the ship and that from the theodolite A being 



not less than 30°. At the instant of firing each gun the ship 



