reille: anti-aircraff firing 



477 



(6) This same method permits one to control or rectify periodi- 

 cally the measurement of the altitude. Thus, suppose the ob- 

 server A operating as has just been said; the signal of observer 

 B announcing the passage of the machine in the next azimuth 

 will be heard: either at the same time as the lower end of the 

 rule crosses the said azimuth drawn on the plotting-board, in 

 which case the altitude has not changed; or before the reaching 

 by the lower end of the rule of the azimuth, in which case the 

 machine has gone higher; or after the passage of the lower end of 

 the rule on the azimuth, in which case the machine has come 

 down. 



Fig. 7. 



In the last two cases the observer A , as soon as he hears the 

 signal, will recall or send back his riile along the sighting plane 

 so as to replace it on the azimuth announced; from this will re- 

 sult the rectification of the reading of the altitude and also the 

 rectification of the horizontal projection of the flying machine. 



(2) The linear velocity may be measured in the following 

 way (fig. 7): 



A given length of a horizontal wire, for instance, 1 cm. repre- 

 senting 100 meters and thus corresponding to a scale of 1/10,000, 

 is stretched across a vertical aperture at such a height above the 

 plane of the board as to represent the altitude of the flying 

 machine (for example, 20 cm. representing 2 kilometers). 



