ANA L YSIS OF Q UA NT IT A Tl VE PHENOMENA . ill 



tion is differently conducted, but the principle is the 

 same ; the telescope is fixed to the circle, which moves 

 with it, and the angle through which it moves is read 

 off at three, six, or more points, disposed of at equal 

 intervals round the circle. The older astronomers, down 

 even to the time of Flamsteed, were accustomed to use 

 portions only of a divided circle, generally quadrants, and 

 Homer made a vast improvement when he introduced 

 the complete circle. 



The transit circle, employed to determine the meridian 

 passage of heavenly bodies, is so constructed that the 

 telescope and the axis bearing it, in fact the whole moving 

 part of the instrument, can be taken out of the bearing 

 sockets and turned over, so that what was formerly the 

 western pivot becomes the eastern one, and vice, versd. 

 It is impossible that the instrument could have been 

 so perfectly constructed, mounted, and adjusted that the 

 telescope should point exactly to the meridian, but the 

 effect of the reversal is that it will point as much to 

 the west in one position as it does to the east in the 

 other, and the mean result of observations in the two 

 positions must be free from such cause of error. 



The accuracy with which the inclination of the compass 

 needle can be determined depends almost entirely on the 

 method of reversal. The dip needle consists of a bar 

 of magnetized steel, suspended like the beam of a delicate 

 balance on a slender axis passing through the centre of 

 gravity of the bar, so that it is at liberty to rest in that 

 exact degree of inclination in the magnetic meridian 

 which the magnetism of the earth induces. The in- 

 clination is read off upon a vertical divided circle, but 

 to avoid any error in the centring of the needle and 

 circle, both ends are read, and the mean of the results 

 is taken. The whole instrument is now turned carefully 

 round through 180, which gives two new readings, in 



