PARAHYBA DO SUL 



PARALLELOGRAM 



Parahyba do Sul. River of 

 Brazil. It rises in the Serra do Mar 

 in Sao Paulo, and flows N.E. across 

 the state of Rio de Janeiro in a 

 narrow valley to enter the Atlan- 

 tic below Campos, after a course 

 of 500 m. 



Paraldehyde. C 6 H, 2 3 . Colour- 

 less liquid with burning taste. It 

 is used in medicine to produce 

 sleep, and is often employed in 

 asylums as a hypnotic in mania 

 and melancholia. 



Parallax. In astronomy, the 

 apparent displacement of a celes- 

 tial object by a change in the 

 position of the observer. A simple 

 illustration of an apparent dis- 

 placement of this character occurs 

 to a passenger observing a church 

 steeple from a moving train : the 

 steeple may be first S.E., then E., 

 and last N.E. If the distance be- 

 tween the positions of the first and 

 last observations is known, then the 

 distances between these positions 

 and the church steeple can be 

 calculated. For astronomical pur- 

 poses the earth in its yearly revolu- 

 tion resembles the moving train, 

 and thus the measurable apparent 

 shifting of various stars is of great 

 importance as furnishing a method 

 of ascertaining their distances. 

 They are so great that the parallax 

 of a star is obtained by taking as a 

 base line the radius of the earth's 

 orbit. The small angle is found 

 then by measuring the apparent 

 change of place in the position of a 

 star caused by the earth's actual 

 change of place in its journey round 

 the sun. The parallax of the near- 

 est star Alpha Centauri is only 

 75", equivalent to a distance 

 of over four light years. 



Solar parallax is obtained by 

 taking the earth's diameter as a 

 base line. The best way to obtain 

 it is indirectly, by observing the 

 parallax of one of the asteroids, 

 from which the distance of the sun 

 can be easily calculated. Solar 

 parallax by observation of the 

 asteroids has been obtained at 25 

 different observatories, hi order to 

 obtain a result as accurate as 

 possible. It is one of the most 

 important figures in astronomy, as 

 the distance of the sun from the 

 earth is an astronomical measuring 

 line. The asteroids Iris, Sappho, 

 and Victoria were chosen for this 

 combined effort to obtain an 

 accurate result, and the figure 

 8 '802" was obtained, corresponding 

 to a distance 92,874,000 miles be- 

 tween the earth and the sun. Solar 

 parallax has also been calculated 

 from considerations of the aberra- 

 tion of light which causes an 

 apparent shift in the position of 

 an object due to the movement of 

 the earth, and the velocity of light. 



Parallel. In geometry, term 

 used for straight lines in a plane 

 which do not meet however inde- 

 finitely they are produced, i.e. they 

 always remain at the same distance 

 from each other. 



By an extension of the Euclidean 

 definition, it is used for things that 

 are similar, e.g. parallel passages in 

 literature. In gymnastics, parallel 

 bars are two bars used chiefly for 

 hand and arm exercises. A parallel 

 ruler is one which enables one 

 straight line to be drawn parallel to 

 another. In electricity, accumu- 

 lators are said to be connected in 

 parallel when all the positive poles 

 are connected to one wire and all 

 the negative poles to another. 



1673. The main method of attack 

 for the infantry consisted of 

 pushing forward by means of zig- 

 zag trenches in order to breach the 

 walls. The workers on these 

 trenches had to be protected from 

 sorties of the besieged garrison, and 

 for this purpose parallels were 

 arranged at intervals for the 

 accommodation of the guards, and 

 consisted of trenches sited at right 

 angles to the direction of the 

 attacking trenches and parallel to 

 the face of the work being attacked. 

 New parallels were made at 

 intervals as the work progressed, 

 so that the guards were always 

 close-up to the working parties. 



See Fortification. 



Parallel Bars. 



Appliance used 

 i n gymnastics. 

 They consist of a 

 pair of hoi izontal 

 bars supported 



frank' 



Fig. 1. Watt motion applied to a 

 beam engine. See text 



In machinery, parallel motion is a 

 mechanism for producing straight- 

 line motion by means of linkwork. 

 The best known, from which others 

 have been developed, are those in- 

 vented by Watt and Scott Russell. 

 Fig. 1 is the Watt motion, as ap- 

 plied to a beam engine. The beam 

 is supported at C. Link P B rotates 

 about P. A and B are connected 

 by link A B, to which the end of the 

 piston-rod is attached. If A C : P B 

 ::B X : A X the circular motions of 

 B and A will counteract one an- 

 other and X will travel in the line 

 of the piston-rod. 



In the Scott Russell motion 

 (Fig. 2) link P X swings about P 

 and is attached to the middle of 

 barAB. P X=A X = B X. Bis 

 pivoted on the end of the piston- 

 rod, and A is pinned to a block 

 moving in a slide. See Linkages. 



Parallel. Term for a type of 

 trench used when attacking 

 fortresses, introduced by Vauban in 



Parallel 



Trench. Dia- _^ 



gram showing sys- #/**. Dar a.v 

 tern of rarallel 

 trenches introduced by Vauban 



on a framework which allows of 

 their being adjusted to six feet or 

 less from the ground. Supporting 

 himself by a hand on either bar, 

 the gymnast is able to perform 

 various exercises. See Gymnastics ; 

 Physical Training. 



Parallelepiped. Solid figure 

 contained by six parallelograms of 

 which every two opposite faces are 

 parallel. A brick is a rectangular 

 parallelepiped. 



Parallelogram of Forces. In 

 mechanics, a rule for finding the re- 

 sultant of two forces. The rule is as 

 follows. If two forces acting at a 

 point are represented in magni- 

 tude and direction by two straight 

 lines A and B ; 

 they are together 

 equivalent to a 

 single force, repre- 

 sented by the 

 diagonal C of the 

 parallelogram con- 

 structed on the two 

 straight lines pass- 

 ing through the 

 point. The rule also 

 applies to displacements and 

 velocities or any vector quantity. 

 The rule is implied in Newton's 

 second law of motion. See Vector. 



