522 HYDROGRAPHICAL SURVEYING [app.f. 



F. — Base hy Sound, 



2 t f 

 To prove that T = , - / 



Let d be dis'ance iu feet belwcen slalions, 

 V the velocity of sound, 1 in feet per 

 X „ of the wind, f second, 



t the observed interval in seconds with the wind, 

 ti „ against the wind, 



T the mean interval required. 



'Jlieu d = V T. 



By observation d = v t + x t 



and d = V ti — X ti. 



Dividing by t and t^ we get 

 d 



— - V + X, 



d 



— = V — X. 



Adding, we have 

 d . d 





it' 



d — V — , : 



but we have also 



d = v T, 



and by equating 



< + V 



