TRANSIT INSTRUMENT. 



477 



Fig. 512. Ellipse. 



are continued. The comets move in parabolic curves, and consequently do 

 not again come within our vision unless their direction be altered. 



This figure has a long axis, a b (fig. 512); and perpendicular to this a 

 short axis, d e, passing through the centre, c. The two 

 points, s s', are called the foci of the 

 ellipse ; also, as is evident from 

 the construction of the figure, any 

 two lines drawn from the two foci, 

 to any point of the circumference^ 

 for instance, s and S' m, or s ;;/' 

 and s 'm', etc., which represent the 



thread when the pencil is at m or ;//', are together equal to 

 the larger axis of the ellipse. These lines, and we may 

 imagine an infinite number of such, are called radii 

 vectores. The distance of the foci, S or S 7 , from the 

 centre, c, is called the eccentricity of the ellipse. It is 

 evident that the smaller the eccentricity is, the nearer 

 the figure approaches to that of the circle. The super- 

 ficies of the ellipse is found by multiplying the two half 

 axes, ac and dc, by each other, and this product by the 

 number 3*14. 



The Diagonal Scale is shown in the margin. 

 It is used to make diagrams so as to bring the relative 

 distances before the eye. The larger divisions repre- 

 sent, it may be, miles, or any given distance ; the 

 figures on the left side tenths, and the upper range 

 hundredths of a mile. So a measurement from Z to 

 z' will represent two miles, we may say, with so many 

 tenths and hundredths. 



The Transit instrument is due to Roemer, a 

 Danish astronomer. It consists of a telescope so con- 

 structed as always to point to the meridian, and rotates 

 upon a hollow axis, directed cast and west. At one end is a graduated 



circle. The optical axis of the tele- 

 scope must be at exactly right angles 

 to the axis of the instrument, it will 

 then move on the meridian. There is 

 an eye-piece filled with two horizontal 

 and five vertical wires, very fine, the 

 latter at equal distances apart. The 

 star appears, and the time it takes to 

 cross is noted as it passes between 

 each wire, and the mean of all the 

 transits will be the transit on the meri- 



Fig. 514- -Transit instrument. dian. For if WC add the times of 



513- Diagonal scale. 



