XV.] ANALYSIS OF QUANTITATIVE PHENOMENA. 365 



Flamsteed, were accustomed to use portions only of a 

 divided circle, generally quadrants, and Eomer 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 magnetised steel, suspended somewhat 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 inclina- 

 tion is read off upon a vertical divided circle, but to avoid 

 error arising from 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 1 80, which causes the needle to assume a new 

 position relatively to the circle and gives two new readings, 

 in which any error due to the wrong position of the zero 

 of the division will be reversed. As the axis of the needle 

 may not be exactly horizontal, it is now reversed in the 

 same manner as the transit instrument, the end of the axis 

 which formerly pointed east being made to point west, and 

 a new set of four readings is taken. 



Finally, error may arise from the axis not passing 

 accurately through the centre of gravity of the bar, and 

 this error can only be detected and eliminated on chang- 

 ing the magnetic poles of the bar by the application of a 

 strong magnet. The error is thus made to act in opposite 

 directions. To ensure all possible accuracy each reversal 



A A 2 



