PRO 



Nutation in Right Ascension. 



This 8",5 tan. dec, cos. (N A) + 1",1 tan. dec. cos. (N -f A) and 

 if to this be added the equation of the equinoxes, the whole effect of 

 nutation will 



4. 8,"5 tan. dec. cos. (N A) + 1",1 tan. dec. cos. (N -f. A) 4. 

 17", 2 sin. N. 



Equation of the obliquity. 

 This = 9",63 cos. N. 



PRESSURE of Earth against walls. 'See Earth, pressure of. 

 PRESSURE of Fluids. See Fluids. 

 PRESSURE, centre of See Centre. 

 PRISM. See Refraction. 



PROGRESSION, Arithmetical ', Geometrical, and Harmonica!. 

 I. Arithmetical Progression. 



All the cases of Arithmetical progression may be solved by the follow- 

 ing formulae : 



1. Let a first term, I last, b common difference, n number of 

 terms, s = sum of the series ; then 



Of the quantities , /, b t , any three being given, the other may be 

 found by the equation 



I a + n l. b. 



2. Of the quantities a, 6, , .v, any three being given, the other may be 

 found by the equation 



3. Of the quantities , I, n, s t any three being given, the other may be 

 found by the equation 



II. Geometrical Progression. 



1. All the cases of Geometrical Progression may be solved by the fo? 

 lowing formulae : 



Let a = first term, I last, r = common ratio, n = number of terrn^, 

 s = sum of the series ; then 



Of the quantities a, l t r t n t any three being given, the other may be 

 found by the equation 



