370 M. Chacornac on the Moon. 



M. CHACOENAC ON THE MOON.— OCCULTATIONS. 



BY THE KEV. T. W. WEBB, A.M., E.E.A.S. 



Theee lias probably never been a time since the invention of 

 the telescope when selenography has received so much general 

 attention as at the present moment ; and on this account our 

 astronomical readers may be interested in learning the conclu- 

 sions which have been deduced as to the nature of the lunar 

 surface by the eminent French astronomer, M. Chacornac, to 

 whose care has been consigned the great silvered-glass Fou- 

 cault speculum of about 2'f English feet in diameter, and who 

 is carrying on his observations at Ville-Urbanne, near Lyons. 

 What aperture and power he may have used does not appear ; 

 or, indeed, whether this particular instrument was employed 

 in his researches ; but they who have tried the effect of only 

 twelve inches of silvered glass will not readily suppose that the 

 whole of that magnificent mirror could have been used to col- 

 lect the lunar rays. 



This celebrated observer begins by telling us, what was stated 

 many years ago as the experience of English astronomers, that 

 the forms of the lunar craters become less regular when scruti- 

 nized with great optical power; he finds that their structure then 

 appears more similar to that of analogous terrestrial formations 

 than might have been previously supposed, and considers that 

 astronomers may undertake an " orographical" examination of 

 our satellite, as geologists have done of the primary planet. 

 The disadvantage is indeed great of having to contemplate 

 objects at a distance of 240,000 miles, and instead of distin- 

 guishing strata by the microscopic shells which they contain, 

 being obliged to reckon upon 120 yards as the minimum visibile 

 in lunar dimensions. It is, however, somewhat counterbalanced 

 by the opportunity of comparing in a single view the formations 

 of an entire hemisphere. Much may be learnt from shadow, 

 as to the minuter details. There are steppes in Central Asia, 

 where, from the uniformity and whiteness of the soil, the 

 shadow of a man at sunset visibly lengthens his stature one 

 hundredfold ; and such is the case on the moon. No sensible 

 penumbra attends the shadows there, and owing to the absence 

 of reflected light from an atmosphere, they are clearly defined, 

 and, under a very low angle of illumination, give us intelligence 

 of many irregularities of form, too minute for direct observa- 

 tion. By such an examination we become acquainted with the 

 fact, that the lunar surface falls naturally into two contrasted 

 divisions. The surface in relief, which has always been 



