590 Meeting of the British Association. [Oct., 



on De la Kue's, but not a vestige can be discovered on Eutberford's. 

 Some attention bas also been given to aj^parent clianges of brigbt- 

 ness and tint, a subject to wbicb Webb called attention a few years 

 since. Tbree or four somewbat conspicuous spots were adduced as 

 exhibiting these alternations, wbicb appear to be independent of any 

 agencies with wliicb we are acquainted. 



l\Ir. W. E. Birt then read a paper " On Secular Variations of 

 Lunar Tints." The author, in alluding to the importance of an 

 examination of the tints of the lunar surface, remarked that to 

 carry it out in its entirety would entail a most enormous labour, 

 but he recommended that a few of the most promment objects 

 should be selected for the purpose. He called attention to the 

 fact that changes of tint and brilhancy were common on the moon. 

 These changes were generally referred to variations in the angle 

 at which the sun's hght fell upon the object. The author then 

 proceeded to notice certain differences which have been observed 

 between the tints of objects as recently determined, and those 

 of the same objects as recorded by previous selenogi-aphers. We 

 have the means of obtaining presumptive evidence of change of 

 tint, independent of illumination, in the fact that if any two spots 

 be taken, one being brighter than the other at a given epoch of 

 illumination, should the order of brightness be reversed at any sub- 

 sequent epoch, the illumination being the same, the legitimate con- 

 clusion would be that a change had occurred in the meantime ; and, 

 as being unconnected with any theoretical considerations of change, 

 the autbor suggested the term " secular variation of tint " to desig- 

 nate such phenomena. In the concluding part of the paper the 

 author referred to the number of spots which have been observed 

 during the last forty-nine years on the lunar crater Plato. 



In a communication " On the Lunar Crater Plato," W. E. Bui 

 stated that certain peaks on the western waU of the crater had been 

 measured by Beer and Maedler, the heights varying from 5000 to 

 7000 English feet. These peaks at sunrise cast well-defined long 

 shadows on the floor, and these shadows bad been measured by 

 Professor Chalhs, of Cambridge. Mr. Birt compared dehneatious 

 of the shadows by Professor Challis, the late Lord Eosse, the late 

 Eev. W. E. Dawes, and J. Birmingham, Esq., of JMillbrook Tuam, 

 and finds some interesting results, among which may be named the 

 proximity of the shadows of the three principal peaks to three very 

 minute craters on the floor of Plato, thus furnishing a means of 

 readily identifying these craters at any future time. 



Mr. G. J. Symons presented the report of the committee on 

 Underground Temperatures. The committee had tried experi- 

 ments on underground temperatures at Glasgow, Dundee, and 

 wherever they could get access to very deep wcEs or borings in the 



