ON THE PHYSICAL ASPECT OF THE MOON, 



183 



Fig. 1. 



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^^ 



Key-plan of Plato, from observations by W. R. Birt, F.R.A.S., between January 5, 1860, 



and October 19, 1861. 



I. n. — A short range of mountains running at first nearly at right angles 

 to the mountainous rim of Plato, from a break in tlie northern or, rather, 

 north-western portion of the rim. This range of mountains is of a curved 

 form, and terminates in the mountain i^. It constitutes the western rim of 

 a crateriform formation to the north of Plato. 



This mountain- range has been tlie subject of eleven observations between 

 January 1860 and May 1862. Schroter had previously observed it, and 

 marked it /<. Under a suitable illumination, a shallow depression is seen 

 westward of this mountain-range, the land rising a little on the westward of 

 it, so that a somewhat narrow valley is enclosed between the two. There are 

 two well-defined peaks on the eastern or highest range, and a small one be- 

 tween them and the rim. 



II. /. — A break on the north-western rim of Plato, which is doubtless the 

 continuation of the narrow valley west of the mountain-range (?i). It is 

 distant about 0*75 of the longest diameter of the apparent ellipse from the 

 east, and is very distinctly shown in the drawing of Schroter. 



The observations of this break in the rim of Plato have been numerous. 

 On three occasions the valley-like character of it has been recorded. Under 

 a suitable illumination, u bright streak from Anaxagoras to Plato may be 

 seen terminating near this break. 



III. m. — A bright spot on the north-west portion of the rim, close to and 

 cast of the valley (0- On the 28th of May, 1860, I have recorded a high 

 alpine mountain in the locality of this spot. • ' 



This bright spot has been observed on nine occasions, and on one occasion 

 as a dusky spot. 



IV. — The interior slope of the north and north-cast border. This slope 

 undergoes variations of luminosity, according as the incidence of the solar 

 rays vary ; it has two dark oval markings. 



V. 2.' — Under a somewhat late illumination, 21*5 days moon's age, the 

 rim of this part of Plato presents the ayjpcarance of a sharp angle in the 

 neighbourhood of the westernmost of the two oval markings, and from this 

 point an irregularly formed crag overhangs the slope. This crag has also 

 been seen under the morning illumination. 



There are strong indications of a circular range of mountains existing on 

 the north of Plato, of which the range (w) forms the western side : the in- 

 cluded area is crossed by two dark but narrow lines, which appear tp be of 



