272 PROFESSOR AIRY ON THE LATITUDE 



A single observation therefore gives us no tangible result. It is 

 necessary to have one other observation, or a series of observations, 

 by which the reading of that point of the limb can be found which 

 is opposite to the same index when the telescope is directed to some 

 point of reference; then the difference between this reading and the 

 former is the angular distance of the object observed from the point 

 of reference. It was intended originally by the maker that this point 

 of reference should be the celestial pole. In practice, however, it is 

 found necessary to descend one step nearer to terrestrial things, and to 

 adopt for the point of reference the zenith ; a point which, though not 

 marked any more than the pole by any obvious phenomena, can yet 

 be discovered by a process which involves less of astronomical assump- 

 tions, and requires a shorter time for the complete determination. 



The method of determining the zenith point from observations 

 by reflexion at the surface of mercury, has been introduced into 

 observatories almost entirely by the practice of the present Astronomer 

 Royal at the Greenwich Observatory. The use of two similar circles 

 (as at Greenwich) makes the process one of little labour, though requiring 

 the co-operation of two observers. The same celestial objects being 

 repeatedly observed by direct vision with both circles, the differences 

 of the corresponding readings of the two circles are found ; and any 

 observations made with one can be referred to the other. Then when 

 any bright star passes the meridian, one circle is employed in observing 

 it by direct vision, and the other at the same time is employed in 

 observing it by reflexion at the surface of mercury ; the reading of the 

 latter circle is referred to the former circle; and then the reading 

 which is a mean between the reading for the direct observation and 

 the referred reading for the reflected observation, is the reading that 

 corresponds to a horizontal position of the telescope; and by adding 

 or subtracting a quadrant, the reading which corresponds to a zenithal 

 position of the telescope is obtained. 



With a single circle this process cannot be adopted. In some 

 instances it has been imitated by observing a star directly on one 

 night, and observing the same star by reflexion on another night. The 



