186 REPORT — 1861. 



The existence of this mountain is well established, having been observed 

 by Schroter, and marked by him D ; by Beer and Madler, and marked by 

 them c ; and by the writer, as above. The chain of mountains is given 

 somewhat difl'erently by each observer, but no doubt can be entertained of 

 its existence. 



XIX, Y. — A very narrow ledge or terrace within the interior of the south- 

 west border of Plato, appearing as a lucid fringe when the shadow of the 

 summit of the border is sufficiently narrow to allow of the illumination of 

 the floor of the terrace. See fig. 2, in which 



Fig. 2. 



Section of the south-west interior slope of Tlato, the Hartvrell Ledge, from observations 



by W. R. Birt, F.R.A.S. 



F. Represents the floor of the crater. 

 S — S. The south-west interior slope. 



Y. The terrace or ledge. 



a. The summit of the slope. 



Z. A ravine exterior to the crater. 



S — S. The incident ray when the ledge is in deep shadow, the entire floor 

 being illuminated. 



S' — S'. The incident ray when the ledge is partly illuminated. 



S" — S". The incident ray when it is wholly so. 



On May 18, 7 0, 1861, I observed the interior shadow of the western rim 

 to fine off" on the south-west side. It presented the appearance of a very fine 

 line, with two bright spots, as if there were two small mountains on the ledge 

 or terrace. With Dr. Lee's permission, I propose to designate this terrace 

 the Hartwell Ledge. 



This ledge has been observed on seven occasions. 



XX. aa. — The sunmiit of the south-west slope ; observed on Jive occasions. 



XXI. Z. — A ravine on the surface exterior to Plato ; observed on thirteen 

 occasions. 



XXII. y. — A liigh peak on the south-west wall, recognized in the early 

 morning illumination by its long shadow stretching far along the floor ; 

 observed on six occasions. Schroter has figured the shadow. 



XXIII. I. — A high peak on the west wall, recognized as above, and 

 figured by Schroter; observed on tico occasions. 



XXIV. e. — A similar peak on the north-west wall, also figured by Schroter, 

 and observed livice. 



These three peaks occasion at sunrise a well-marked indented shadow, 



