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



Mond, p. 267 ; they are scarcely, if at all, perceptible in the 

 new edition of the map. Space and time are important elements 

 in unravelling the complexity of natural phenomena; and in the 

 more extensive basis furnished by the observations of twenty 

 lunations, it is essential to combine the two in order to arrive at 

 a just appreciation of the results. Plate IV. contains the more 

 important curves obtained in the course of the twenty lunations; 

 and while they necessarily present the features included in the 

 curves of Plate III., the additional portions furnish evidence of 

 the operation of agencies confined apparently to certain localities, 

 and which accordingly cannot be referred to such as are extra- 

 neous to the moon ; for example, illumination, reflection, &c. 



On Plate IV. may be found several pairs of similar curves, 

 the most striking pair being that of Nos. 19 and 13. These 

 spots, with No. 16, are situated on a somewhat bright curved 

 streak on the north-west part of the floor. The peculiarities of 

 the curves of these spots consist of the bold maxima of August 

 and September 1869, which signify that in those lunations the 

 spots were very frequently seen, much more so than either pre- 

 viously or since. Another feature is the rarity of visibility of 

 the spots Nos. 19 and 13 during the last six lunations. The 

 curve of the neighbouring spot No. 16 is not characterized by 

 this rarity of visibility ; on the contrary, it actually increased, 

 and the spot was seen more frequently at the time when the 

 minima of Nos. 19 and 13 occurred. The spot whose curve most 

 nearly agrees with that of No. 16 is No. 5. It will be seen from 

 Plate IV. that, with only one exception (October and November 

 1870), the inflexions of the curves are similar, but the range in 

 the case of No. 16 is less. The logical inference is that the va- 

 riations of visibility of the two spots were produced by the same 

 agency, or at all events by similar agencies. 



The most remarkable circumstance connected with the curve 

 of No. 16 is its departure from the type of its neighbour No. 19, 

 and its conformity to the curve of No. 5. It is difficult to un- 

 derstand this unless we take into consideration the structure of 

 Plato. Crossing the plain from N.W. to S.E. is a fault which 

 has dislocated the border in two opposite points, each marked by 

 a gap or depression in the wall. Now the spots Nos. 5 and 16 

 are not far removed from this fault on the west ; and if we were 

 to admit a local action connected with this fault, we might pro- 

 bably obtain an explanation of the coincidence of the forms of 

 the two curves furnished by spots so widely separated as Nos. 16 

 and 5. 



A third pair of similar curves are those of Nos. 22 and 7, still 

 more widely separated than Nos. 16 and 5, No. 7 being near 

 the east border, and No. 22 near the west border. The spots are 



