P A R 



2. The angles oi' incidence and reflexion are in the same plane ano 

 >qual. 



The angles of inrideuee and refraction are in the same plane, and their 

 sines bear an invariable ratio to one another for the .same medium. 



For the various subjects connected with this branch of science, see the 

 respective heads. 



OSCILLATION, Centre of See Centre. 



PALLAS. 



This planet was discovered by Dr Olbers, of Bremen, March 28, ISCtt. 

 For its elements see Planets, elements of. 



PARABOLA, principal properties of. See Conic Section*. 



PARACENTRIC velocity. See Central Forces. 



PARALLAX ( Woodkoitse, Play fair.) 



1. If P be the horizontal parallax of a heavenly body, p the parallax 

 at a zenith distance z. 



p P X sin. z. 



Cor. If R be the radius of the earth j r the tubular radius, d the dis- 

 tance of the body, then 



d = X R. 



To adapt this to computation, r must be expressed in degrees, minutes, 

 &c. then 



2. If two observers under the same meridian, but at a great distance 

 from one another, observe the zenith distances of the same planet, when 

 it passes the meridian on the same day, they can from thence determine 

 the horizontal parallax. 



Let L and L' be the two latitudes, z and z' the observed zenith dis- 

 tances, then 



sin. # 4- sin, ' 

 198 



