io8 IN STARR Y REALMS. 



siderable but not of the largest dimensions, mounted in a 

 particular way for a particular purpose. A view of the 

 meridian circle at Dunsink Observatory is given in the 

 adjoining sketch. The telescope is attached to an axis 

 about which it can revolve, just as a cannon can be 

 turned up or down in its bearings. The axis carries 

 two large circles. On these the utmost refinements 

 of mechanical skill must be exercised. The circles 

 in the instrument at Dunsink Observatory, the estab- 

 lishment over which I have had the honour to preside, are 

 about three feet in diameter. There is a rim of silver 

 let into the margin of the wheel ; this silver is graduated 

 by fine marks marks indeed so fine that they require a 

 microscope for their detection. When the telescope is 

 pointed to a star, the position of the marks shows the 

 elevation of the instrument, and thus the height of the 

 star above the horiaon is determined. The telescope is 

 carefully adjusted so as to move in the plane of the 

 meridian. It can be pointed to the due north, then it 

 can be raised up to the point of the heavens vertically 

 overhead, then it can be turned downwards to the south. 

 But it is of the essence of the meridian circle that it 

 admits of no other movements ; it cannot be turned to the 

 east or to the west in the slightest degree. 



A star or a planet, the sun or the moon, can there- 

 fore only be observed with the meridian circle at the 

 very instant of crossing the meridian. In fact, one of 

 the purposes of observing with the meridian circle is 

 to determine the time at which the celestial body is 

 on the meridian. To obtain the requisite accuracy, it 

 will not be sufficient merely to observe the star in 



