TRANSACTIONS OF THE SECTIONS. 17 



after a lapse of years that a well-conducted series of observations will determine 

 if there he secular chano^e in these stripes and tints. Should some disappear, and 

 others not observed before become visible, tlie presumptive evidence is that a secular 

 change has taken place. This appears to be the case -with several spots on Plato. 

 The earliest record that I have been able to find of any well-defined small spots on 

 the floor is that of Clruithuisen, who observed eight ; later records furnish evidence 

 of eighteen additional spots having been observed. Of these, nine have been added 

 so recently as last February and since. Of the twenty-sLx recorded, six of the 

 earliest observed are still visible, and of the nine seen since January last five have 

 been more or less constantly seen by two observers, leaving fifteen which have 

 either become invisible or are very rarely seen. 



When the sun is rising upon Plato the shadows of the peaks on the western wall 

 are admirably calculated to identify certain of these spots, and probably of deciding 

 between craters opened on tlie floor of Plato and those small white spots to which 

 Herr Tempel solicits attention as indicative of a tvann chemical activity, and which 

 appear mider a high solar illumination, there being no indication whatever of cra- 

 ters existing on their sites as the terminator passes them. 



Beer and Madler measured the three peaks on the western wall, viz. y, d, and 

 e, the heights being as under : — 



y=72o8 Eng. feet, 8=6369 Eng. feet, e = 5128 Eng. feet. 



Challis measured the shadows on May 16, 1853. The measures were made 

 parallel and perpendicular to a line coincident with the longest diameter of Plato. 

 At the time Challis observed Plato nine spots had been recorded as having been 

 seen on the floor, four of which were delineated by Challis, numbered 1, 3, 4, and 5. 

 They have since been observed by Eosse, Dobie, Dawes, and Pratt, and some of 

 them by Knott and other observers. 



Rosse in 1862, Dawes in 1863, and Birmingham in 1869, observed the shadow 

 of the peak y in close proximity with No. 5 (?). Rosse and Birmingham have drawn 

 No. 1 with the shadow of S just receding from it, and Binningham gives No. 3 at 

 the) extremity of the shadow of the northern peak e. As matter of observation 

 these are important ; for by the variations of the seasons at the moon the shadows 

 vnll fall somewhat diflerently in summer than in winter ; the extreme range is, 

 however, but small, the azimuthal angle with the same altitude not exceeding 3° ; 

 and as solar azimuths at the moon are easily calculated, no real difficulty exists in 

 identifying these spots as lying near the shadows of the peaks. The peaks them- 

 selves merit attention ; Challis's shadow of S terminates by a straight line ; he 

 measured the two extremities of this line. Rosse delineates the termination of the 

 shadow as from two pinnacles upon the summit, with the crater No. 1 between them. 

 Birmingham gives the south pinnacle only, with the crater No. 1 just beyond it. In 

 Dawes's drawing No. 1 lies between the shadows of -y and 8. There are other peculi- 

 arities about the shadows which require the solar azimuths to be calculated for illus- 

 trating them. It is to be hoped that good drawings, accompanied by descriptions of 

 the shadows of the west wall and craters visible on the floor, vdll be midtiplied, as 

 in referring such drawings and descriptions to the respective periods at which they 

 were made in the luni-solar year, we may become better acquainted with the nature 

 of the summit of the wall, and by means of large apertures, small craters on the 

 floor, hitherto overlooked, may be detected. 



Although this branch of lunar physics is confessedly difiicult, it is by no means 

 insurmountable. The greatest drawback consists in the paucity of recorded obser- 

 vations of an earlier date, from which arises the uncertainty of the existence of the 

 more recently obsei'ved spots. The way, however, for precise observation and 

 careful discussion is gradually opening, and we may hope that Schmidt's suggestion 

 of " trying by the aid of more powerful telescopes to represent the topography of 

 the yet undelineated details of the surface of the moon," may bear good li-uit, al- 

 though the further we proceed the more unable are we to see the end of the work ; 

 " it is," says Schmidt, '' as if from the ordinary determination of place of the brighter 

 stars down to the eighth magnitude, one passes on to that of the stars of the Milky 

 Way." So, in like manner, each addition to om- knowledge of lunar physics leads 

 us onward to the study of still more minute forms, in which perhaps lui'k the germs 

 of future interesting and important discoveries. 



1869. 2 



