﻿188 Mr. W. R. Birt on some recent Investigations 



strengthening the evidence; and the spots, intermittent in their 

 visibility but persistent in their existence, may probably be re- 

 ferred to those minor openings found more or less in the neigh- 

 bourhood of fissures. 



The curve of spot No. 14, situated nearly midway between 

 spots Nos. 5 and 22, is the most remarkable of the series, par- 

 taking, on the one hand, of the features of the curves of Nos. 19, 

 13, and 22 in the increase of visibility in August and September 

 1869, and, on the other, of Nos. 5 and 16 in the inflections of 

 the curves during the six lunations February to July inclusive ; 

 it also agrees with the curve of No. 5 in the maximum of August 

 and September 1870. Of all the spots, No. 14 has manifested 

 the greatest variation of visibility. 



Not only are the spots which manifested increased visibility, 

 as stated above, restricted in " locality," but the epoch August 

 and September 1869, at which this increase occurred, is very 

 marked, and decidedly separates the nine spots Nos. 19, 13, 22, 

 7, 14, 5, 16, 12, and 20 from all the others, constituting them 

 a distinct group, as given in Plate IV. It is to be borne in 

 mind that this group is about a fourth part of the whole number 

 of spots yet known on Plato, and the values of the maximum 

 degrees of visibility so great as to preclude the idea that a fa- 

 vourable state of the earth's atmosphere is the only element 

 which contributed to their development. 



A second manifestation of increased visibility occurred in Fe- 

 bruary and March 1870, which, like the former, was restricted in 

 locality. This, however, was not the same, but extended as a band 

 south of spots Nos. 1 and 4 ; and two only at a considerable dis- 

 tance from this band, viz. Nos. 13 and 20 of the first group, were 

 affected, and that in a very subdued degree. The additional 

 spots forming the second group are Nos. 18, 9, 11, 17, 10, and 

 29. The forms of the curves of these spots, given on Plate IV. 

 group II., are suggestive of the spots themselves having been sub- 

 jected either to very different influences from those which affected 

 the spots of the first group ; or, if the same agencies were in 

 operation on both occasions, it is very certain that such agencies 

 had a very restricted range indeed, and were confined to a very 

 small portion of the moon's surface, so small that the effects of 

 changes in the earth's atmosphere must be manifested over a 

 much greater extent of the moon's surface. 



Maxima of visibility of a somewhat subdued character, except 

 in the case of spot No. 5, occurred in August and September 

 1870 over a somewhat larger area than that embraced by group II. 

 On this occasion neither of the spots Nos. 13 and 20 were 

 affected, but Nos, 22 and 7 exhibited increased visibility. Refer- 

 ence has already been made to the similarity of the curves of 



