﻿[ 183 ] 



XXV. On some recent Investigations relative to Lunar Activity. 

 By W. E. Birt, F.R.A.S* 

 [With Two Plates.] 



DURING the last seven years the subject of "Lunar Activity," 

 or the present existence of eruptive action productive of 

 "change " on the moon's surface, has been much agitated. In the 

 summer of 1864 my attention was more particularly directed to 

 it, and I suggested that a rigid and careful examination of the 

 moon's disk should be made, the result being embodied in " a 

 catalogue of lunar objects/' as all previous records were insuffi- 

 cient to determine the question. This, of course, must be a 

 long and laborious work ; but until it is accomplished, at all 

 events for a portion of the moon's surface, it is utterly impossible 

 to decide if an object not previously observed be really " new," 

 or if one presenting a different aspect has undergone a change. 

 This state of affairs was very prominently brought out in the 

 celebrated case of " Linne ; " the evidence was clearly insufficient 

 to establish that a change had really taken place. It was not, 

 however, the vagueness or imperfection of the evidence collected 

 in 1866, 1867, and 1868 that occasioned the failure, but the 

 want of precision which characterized the earlier observations, 

 and the doubt resting upon the accuracy of the earlier drawings 

 and descriptions, the object having been lost sight of during a 

 period of twenty-four years between 1842 and 1866. This 

 doubt still continues. 



In February 1869, Mr. Pratt, of Brighton, a careful observer, 

 sent me a drawing of the floor of " Plato," a walled plain in the 

 northern part of the moon, as seen with his 8J-inch silvered- 

 glass reflector : the drawing contained eleven spots ; previously 

 no more than five or six were known. The positions of these 

 spots differed so greatly from those with which I was acquainted, 

 that I was induced to request that observations of Plato might. 

 be sent to me ; and I have received between March 1869 and 

 December 1870 as many as 1594, contributed by the following 

 gentlemen :— Mr. Crossley of Halifax, Mr. Gledhill of Mr. 

 Crossley's observatory, Mr. Pratt of Brighton, Mr. Elger of 

 Bedford, Mr. Birmingham of Tuam, Mr. Joynson of Liverpool, 

 Mr. Cook of Preston, Mr. Whitley of Truro, Mr. Ormesher of 

 Patricroft, and Messrs. Ingall and Neison of London. 



Having ascertained the positions of the spots observed pre- 

 viously to 1869, one of the first steps was to identify them ; and 

 in the course of that year every one was reobserved and several 

 new ones added (see Plate III., which contains a plan of Plato 

 with the estimated positions of the spots). As the observations 



* Communicated bv the Author. 



