538 



gunpowder fired. Considering that the moon has a great 

 many mountains, it is not astonishing that it should be sub- 

 ject to very frequent storms." In what I observed there 

 was no fulguration, or appearance of trains of luminous mat- 

 ter. They were spots of a steady light, which disappeared 

 after having shone six or eight seconds. They were not 

 reddish, like those Ulloa believed to be the effect of an ex- 

 cavation in the moon. (Phil. Trans. , 1799, p. 116. Meni. 

 de Berlin, 1788, p. 204.) To what cause must we refer these 

 luminous appearances, observed at different times on the 

 lunar disk, during an eclipse of the Sun ? The spots that I 

 remarked beyond the solar disk could not be owing to the 

 same optical illusion, that has made some observers believe 

 they saw a satellite of Venus, in which a few even imagined 

 they perceived phases. 



NOTE B. 



I shall here insert the satisfactory and ingenious explana- 

 tion, that Mr. Arago has given of the phenomenon of twink- 

 ling, and which has not yet been made public. The follow- 

 ing is the note, that this learned philosopher had the kind- 

 ness to write for me. 



**' The natural philosophers and astronomers, who have 

 turned their attention to the twinkling of the stars, have almost 

 all neglected the most remarkable circumstance, perhaps, of 

 the phenomenon. I mean the sudden and frequent changes 

 of colour, by which it is constantly accompanied. The pro- 

 gress, which the physical theory of light has made within 

 these few years, will enable us, I believe, to connect the 

 explanation of this curious fact with the law of interferences, 

 discovered by Dr. Young. 



" According to the experiments of this illustrious philoso- 



