IS THE MOON A DEAD PLANET ? 573 



of observations, made in the years 1869, 1870, and 1871, by six or 

 seven independent observers. 



Proctor has attempted to show that these appearances are the 

 effect of contrast. Thus, at sunrise, the floor of Plato is thrown against 

 a dark background, due to the sombre, barely-illuminated surround- 

 ing regions; while at full moon it has for a background the brilliantly- 

 illuminated surrounding highlands, and should look much darker. 

 But if the great darkening observed to occur in the tint of the interior 

 of Plato is merely apparent, and only what must occur when a dark- 

 ish walled plain is surrounded by a bright background, or, rather, 

 bright environs and this is all Mr. Proctor ascribes to it it must be 

 a perfectly normal occurrence, and the same must take place in every 

 similarly-placed formation, unless that has something anomalous about 

 it to prevent this taking place. All selenographers could instance a 

 number of such walled plains where no such darkening occurs. Are 

 we, then, to assume that these plains possess anomalously constituted 

 interiors, and that only Plato, of all the lunar formations, exhibits the 

 normal phenomena ? This is, of course, entirely inadmissible, and 

 selenographers are thoroughly aware that the effects of contrast al- 

 luded to by Mr. Proctor are entirely incapable of bringing about such 

 an immense darkening in tint as is apparent in the case of the floor 

 of Plato. 



The reason of this singular darkening in the floor of Plato is not 

 yet understood. Selenographers believe that it results from an actual 

 change due to the heating action of the solar rays. But the further 

 elucidation of the subject requires special observations with special 

 appliances. Thus, as in so many selenographical problems, patient 

 observation establishes the existence of certain phenomena, but the 

 elucidation of the meaning of the phenomena established is checked 

 for want of special observations that are never made. For these, 

 selenographers have to appeal to those astronomers devoted to what 

 has been termed astronomical physics, but they are too much engaged 

 on more fascinating subjects to give attention to such inquiries. 



The instances here dealt with will show that selenographers are 

 not without strong evidence in favor of the opinion that has long been 

 unanimously held by them, that processes of change are still actively 

 at work upon the moon. It must not, however, be supposed that the 

 above are the sole instances which they have recognized, because this 

 is by no means true, only the difficulties in establishing others have 

 led to their omission here. Dealing with a subject such as selenogra- 

 phy, it is only those who are familiar with all its details who properly 

 appreciate the evidence in favor of or against its problems. The 

 difficulties in the way of making the true bearings of selenographical 

 questions properly understood are greater than might be imagined ; 

 for even the very elementary fact that volcanic changes such as are 

 now active on the earth would not be recognizable on the moon, in 



