340 Rev, Mr, EmmetVs [Nov. 



of the observer : on the eai tli, the waters of the ocean are per- 

 fectly clear ; the bed of the sea is illuminated by the sun ; but 

 of the light which falls upon water, part is reflected; of the rays 

 which pass below the surface, many are absorbed ; and of those 

 which are reflected from the bed of the sea, many are absorbed 

 by passing through a great extent of water : therefore the bed 

 of the sea will receive and reflect fewer rays than the surface of 

 the moon ; hence seas will appear darker than the land. Again, 

 when the line which separates the illuminated from the dark 

 hemisphere, passes over the bright parts of the moon, it is 

 rugged and broken; the shadows of elevated parts are very 

 black and sharply defined ; but when it crosses the dark parts, 

 the phaenomena are very different ; the line is in general a 

 smooth elliptical curve; it is shaded off'very gradually, whereby 

 it almost imperceptibly terminates in perfect darkness. There 

 are irregularities in it, but they are of a different form to those 

 which present themselves on the bright part of the disc ; these 

 irregularities are of the following sorts : 1. Long, narrow, dusky 

 ridges ; smoothly and softly defined ; not broken angularly ; 

 windings always rounded ; they have often branches ; some- 

 times one side has a whitish brilliancy ; in this case, a shadow 

 is projected by the other, which shadow is very gradually 

 softened at its margin. These ridges are very distinct and 

 numerous in Mare Mediterraneum, when the moon is at the ages 

 represented in maps 13, 14, 15, of Hevehus's Selenographia. 

 2. Bright spots which project considerable shadows. If these 

 be carefully examined, they will be found to be very consider- 

 able elevations : the shadows are remarkably soft, and destitute 

 of the sharpness of those on the brighter parts : if these eleva- 

 tions have cavities within them, the shadow of the margin, 

 which is projected into the cavity, is sometimes very intense and 

 sharply defined ; the altitude of such spots is great. These 

 may be seen at the ages represented in the following maps of 

 the Selenographia ; between 7 and 8 in Pontus Euxinus ; in 

 12, 13, 14, these phaenomena may be observed in M. Christi, 

 J. Ebissus, J. Minorca; in 16 in Corsica and Sardinia, and in 

 many other parts. 3. Spots which have less brilliancy than the 

 part of the moon, called land, by Hevelius, and greater than that 

 which he supposes to be water. Some such appear in Palus 

 Moeotis ; Ericntini Scopuli in Pontus Euxinus are remarkable ; 

 at the age, map 14 to 17, many such may be seen in Pontus 

 Euxinus, Mare Mediterraneum, Mare Hyperboreorum, Mare 

 Pamphilium, &c.: most of these project the soft shadows before 

 named.- A tolerably good idea of the difference between the 

 effect of the bright and dark parts of the lunar disc may be 

 formed by examining Palus McBotis in map 2 ; Pontus Euxinus 

 and S. Extremus Ponti in 6; Pontus Euxinus in 7, 8, 9 ; Mare 

 Hyperboreorum, Mare Mediterraneum, and Mare Adriaticum in 

 12 ; Mare Mediterraneum in 15, 32, 34, 35, 36 ; but if the moon 



