ASTRONOMICAL OBSERVATORIES. 



221 



zenith. The dome rests on six twenty-four pound can- 

 non-balls, which turn between two cast-iron annular 

 grooves. 



In the two flank towers, meridian observing slits are 

 made, about 20 inches in the clear. These begin about 

 two and a half feet from the floor, and extend through 



