OF THE UNITED STATES. 359 



semicircle serving north of the zenith, and the other soutli 

 of it. 



The level for the axis of the transit is a I'rec level hanging 

 between the pillars, and has a tube of upwards of an inch in 

 diameter. 



The illumination is through the axis, by a lantern placed 

 on one of the stone pillars. 



To see the meridian mark distinctly and without parallax, 

 in case of its being somewhat near, (as the nice adjustment 

 will not permit iis to alter the focus from the infinite distance) 

 it has been usual to adapt before the object glass another 

 glass of the focal distance equal to the distance of the mark. 

 I considered this method liable to some objections, and be- 

 sides could not know tlie distances of the future meridian 

 marks. 



I suggested therefore the following simple arrangement, 

 the correctness of which [ had long ago tried, and which ob- 

 tained the approbation of Mr. Troughton. A brass plate is 

 screwed to the end of thr. additinnal tiil)p plarpH before the 

 object glass, having in its centre a hole of not more than half 

 an inch in diameter tlirough which only the middle rays of 

 light are admitted. Thus all parallax is avoided, the image 

 is exceedingly well defined, and the great loss of light which 

 naturally takes place is of no importance. 



As these transits are not within my reach, it would be im- 

 proper to enter into a more minute description of them. 

 This task is left for the astronomer to whose care they shall 

 be committed. 



On the Astrononiical Clocks intended for the Observatories. 



About a year before 1 came to Lonilon, a new clock had 

 been put up in the Royal Observatory of Greenwich, to serve 



