LABOURS RELATIVE TO THE SOLAR SYSTEM. 291 



has been so unjustly assigned as the characteristic of the 

 illustrious astronomer. 



At the close of 1787, Herschel presented a memoir to 

 the Royal Society, the title of which must have made a 

 strong impression on people's imaginations. The author 

 therein relates that on the 19th of April, 1787, h had 

 observed in the non-illuminated part of the moon, that 

 is, in the then dark portion, three volcanoes in a state of 

 ignition. Two of these volcanoes appeared to be on the 

 decline, the other appeared to be active. Such was then 

 Herschel's conviction of the reality of the phenomenon, 

 that the next morning he wrote thus of his first observa- 

 tion : " The volcano burns with more violence than last 

 night." The real diameter of the volcanic light was 

 5000 metres (16,400 English feet). Its intensity ap- 

 peared very superior to that of the nucleus of a comet 

 then in apparition. The observer added : " The objects 

 - ..uated near the crater are feebly illuminated by the 

 Tight that emanates from it." Herschel concludes thus : 

 "In short, this eruption very much resembles the one I 

 witnessed on the 4th of May, 1783." 



How happens it, after such exact observations, that 

 \v astronomers now admit the existence of active vol- 

 canoes in the moon ? I will explain this singularity in 

 a few words. 



The various parts of our satellite are not all equally 

 reflecting. Here, it may depend on the form, elsewhere, 

 on the nature of the materials. Those persons who have 

 examined the moon with telescopes, know how very con- 

 siderable the difference arising from these two causes 

 may be, how much brighter one point of the moon some- 

 times is than those around it. Now, it is quite evident 

 that the relations of intensity between the faint parts 



