424 Count Paul de Saint- Robert on Atmospheric Refraction. 

 where 



otar — sid^ v 

 c _ «gr — sin 2 ^ 

 sin 2 6 cos V 



_ 2*Po , 

 l+4Jc Po 



2po\Po~P / 



4ft [>] =0-00058856 at 0° C., 



m 76 pressure, 45° lat. (Biot.) 



6 zenith-distance of the signal. 



r , r distance of the stations from the earth's centre. 



v angle between the radii r Q , r. 



Earth's radius at lat. 45° 6,366,786 m = 20,888,782 feet. (Arago.) 



Example. Measurement of Monviso above Turin. Data ex- 

 tracted from Mesure d'un Arc du ParallUe Moyen, vol. ii. p. 384. 



Zenith-distance of Monviso seen from Turin . # = 87° 8' 13" 

 Distance from Turin to Monviso . . . . v =0° 34' 53" 

 Earth's radius at Turin r = 6,366,273 m 



. j Barometer m -7464 



iurin ^Thermometer in open air . . 10°'6 C. 



In the absence of simultaneous meteorological observations at 

 the top of Monviso, I shall determine, merely for the sake of 

 illustration, the constant a, viz. the rate of decrease of the den- 

 sity of the air, by means of the simultaneous observations at 

 Verzuolo and Monviso above-mentioned. From them we deduce 



a = 0-000082941. 



With these data our formulae give 



4kp = 0-00055455, a = 0-00027712, 



ctar = 0-14633, 



V=-3°21' 15", 



C= -0-85477, 



# = 3510 m -4. 



Monviso above Turin r — r = 3512*3 metres. 

 Turin altitude .... 273 



Monviso above the sea . . 3785*3 » 



