&2 Mare Vaporum and the Lunar Clefts. 



MARE VAPORUM AND THE LUNAR CLEFTS— 

 OCCULTATIONS. 



BY THE REV. T. W. WEBB, A.M., F.K.A.S. 



With the object of comprehending in a continuous description 

 the whole extent of the Lunar Apennines, our guides have 

 carried us considerably beyond the 1st Meridian, and into the 

 2nd Quadrant of the Moon. They now bring us back towards 

 the W. to complete the Quadrant with which they commenced, 

 and introduce us to the Mare Vaporum (F in our map) . This 

 is a level surface, of a lighter tone than the other Maria, and 

 possessing no very definite boundary, but lying in a general 

 sense between the S.W. slope of the Apennines and the crater 

 Agrippa (26). There would be little to make the region 

 worthy of especial notice were it not that its S. part contains 

 a full development of one of the most curious of the lunar 

 features, the system of clefts (or Rills). Fortunately for the 

 telescopic observer, this district, while it lies in so central a 

 position as to be very little affected by foreshortening, contains 

 two of the largest and most obvious of these mysterious forma- 

 tions, "in which," say B. and M., "an attentive observation 

 can recognize something more than their mere existence." 

 Before we proceed, however, to describe them, it may be pre- 

 mised that there is a difficulty, not merely in comprehending 

 their nature, but even in providing for them a suitable name. 

 This arises from the fact that we are so little familiar with 

 anything analogous to them on the surface of our own globe. It 

 is true that features do exist, which, viewed at a corresponding 

 distance, might have a somewhat similar aspect. We might 

 refer to the transverse fractures of the Balkan range, and of 

 some of the chains of Greece, the Barrancas of Mexico, or 

 the marvellous camns of the Colorado River (described in Int. 

 Obs. iv. 309), which may, perhaps, exhibit as great a similarity 

 to the clefts of our satellite as the terrestrial does to the lunar 

 crater ; aud more could not in fairness be expected. But still, 

 the fissures of our globe are too exceptional to constitute a 

 system such as obtains on the moon, and consequently to have 

 received any generally accepted name. Astronomers, there- 

 fore, are somewhat at a loss how to designate the lunar 

 crack, if such it may be termed. Schr., the first discoverer of 

 these objects, in 1788, called them "canals," or "rills": the 

 former term is obviously unsuitable to a dry surface, and has 

 gone out of use ; the latter has held its ground among the 

 German observers, and very deservedly so, as far as their lan- 

 guage is concerned, in which it signifies a small furrow, or 



