44 Dr. Schroder van der Kolk on the Velocity of Sound. 



1826, vol. ii. p. 22), where the effect of wind is calculated as 

 follows : — A and B are the two c 



stations, C E and D F indicate - e 



the direction of the wind. 



Now let sound travel through 

 still air from A to C in the time 



t, and in the same time let the / 



particle of air C be carried by D A 



the wind to B ; an observer at B will then hear the sound from 

 A at the end of the time t ; and in like manner an observer at 

 A will hear the sound from B at the end of the time t ! , if during 

 this time the sound reaches D, and the wind in the same time 

 carries the particle D to A. 



Let s be the velocity of sound in still air, 

 v the velocity of the wind, 

 AB = a , 

 and ZBAF = <£. 



ThenAC = s/, CB=^, BD = st t , AV = vt'. 



Now the triangles give us 



s 2 * B = a s + w*/ 2 -2av/cos<k (1) 



sH ! * = u* + vH' 2 + 2civt , cos(f>; (2) 



whence, eliminating s 2 , we have 



_ *(t'-t) 

 V ~2tt'cos<£ ; V" \ ' { j 



a, t, t', and <f> are known quantities, and accordingly the velocity 

 of the wind can be calculated from this equation. By substitu- 

 tion in (1) or (2), we then get 



s= VF3S5 f 



All the observations have been recalculated by this formula. 



The direction of the wind is recorded at both stations for 

 every observation in divisions of the wind-rose. The azimuth 

 of the base-line, in the direction from Zevenboomen to Koolt- 

 jesberg, was 32° 30' 55"'8 West of North. The angle <j> is ac- 

 cordingly known. 



When there was any appreciable difference between the direc- 

 tions of the wind at the two stations, the mean direction was 

 taken. 



In this manner we find, for the 27th of June, the following 

 determinations : — 



