476 



SCIENTIFIC RECREATIONS. 



apparatus for measuring angles. 

 like the hand of a clock, and the 

 ment is used by gazing first at one 

 the two objects, between which the 

 angle we wish to determine is made 

 like the church steeples (fig. 508) for 

 instance. The centre of the instrument 

 is placed upon the spot where lines, if 



An index hand, RR, moves round a dial 

 instru- 

 of 



. 



ft*----::;""" 



Jv 



Fig. 508. Determination of distance. 



Fig. sop-Measuring angles. 



-B -h 



drawn from the eye to each of the objects, would intersect. The index 



hand is then put at o, and in a 

 line between the observer and the 

 A object, A. Then the index is 

 moved into a similar position to- 

 wards B, and when in line with it 

 the numbers of degrees passed 

 '""" over (in this imaginary case 20), 

 shows the magnitude of the angle. 

 The simple quadrant is 

 shown in the cut (fig. 5 i o). This 

 was so arranged that when any object in the horizon was being looked at 

 through the telescope attached, a plummet line is at o. But if the 

 telescope be raised to C S, the quadrant will 

 move, and the line will mark a certain 

 number of degrees of the angle which a line 

 if drawn from the star makes with the line 

 of the horizon. The "Astronomical Quad- 

 rants " are as shown in fig. 516, and con- 

 sist of a quadrant of wood strengthened and 

 fitted with a telescope. The circle is 

 graduated on the outer edge, and a " vernier " 

 is attached. The time is determined by the 

 observation of the altitude of a star, and then 

 by calculation finding out at what time the 

 star would have the observed altitude. The 

 quadrant is now superseded by circular in- 

 struments. 



An ellipse is a flattened circle, or oval, 

 and will be understood from the diagrams. 



Let us fix two pegs upon a sheet of paper, and take 

 a thread longer than the distance between the pegs ; 

 draw with the pencil controlled by the thread a figure, 

 keeping the thread tight. We shall thus describe an 

 oval, or ellipse. The orbit of nearly all the heavenly 

 Fig. 5 iT.-E:iip Se . bodies is an ellipse. The parabola is another curved 



line, but its ends never meet; they become more and more distant as they 



Fig. 510 The quadrant. 



