ECLIPSES. 83 



antly the most remarkable and interesting is a central eclipse of the sun. If 

 it be total, the spectacle it offers is most imposing : the light of dav is grad- 

 ually withdrawn to such a degree that the brighter planets, such as Venus and 

 Jupiter, and the stars of the first magnitude, become visible to the naked eye. 

 We see, however, a faint light of the sun behind the disk of the moon. Some- 

 times, as has been stated, when the apparent magnitude of the moon is a little 

 less than that of the sun, the disk of the moon conceals the entire disk of the 

 sun, except only a thin luminous ring surrounding it. This is a phenomenon 

 of very rare occurrence, and only to be seen at particular places on the earth. 

 An instance of it occurred on the 7th of September, 1820. It commenced to 

 be visible at the north latitude of 80, in Hudson's bay, near the eastern coast 

 of New North Wales. It was visible next in the direction of the northeast 

 of Greenland, at the mouth of the Wesel, at Bremen, in the gulf of Venice, and 

 in Arabia deserta, and ceased near the Persian gulf. While this eclipse was 

 produced in these different places, the observers who were on the same me- 

 ridians, but further south, saw only a partial eclipse, and others, still further 

 south, saw no eclipse at all. the contrary took place with observers on the same 

 meridians farther north, to all of whom the eclipse was annular. 



It was during a phenomenon of this kind that Schroter imagined he saw the 

 solar light coming through an immense opening in the moon. Other observers, 

 however, who saw, or imagined they saw, luminous spots on the dark hemi- 

 sphere of the moon, in a solar eclipse, ascribed them to lunar volcanoes. As 

 to the existence of these luminous spots on the dark hemisphere of the moon, 

 rendered manifest in a total eclipse of the sun, we have the testimony of so 

 many astronomers, among whom, besides Schroter, may be mentioned Sir 

 William Herschel and Kater, that we can scarcely doubt their reality. The 

 causes which may produce them have only been explained in the two ways 

 above mentioned, namely, either by the supposed existence of active volcanoes, 

 on the moon, or perforations through the moon, through which the sun's light 

 passes. 



The following description of a total eclipse of the sun. given by Halley, who 

 observed it, is quoted by Arago, and will be read with interest : 



" I send you, according to promise, my observations of the solar eclipse, 

 though I fear they will not be of much use to you. Not being furnished with 

 the necessary instruments for measuring time, I confined my views to examin- 

 ing the spectacle presented by nature under such extraordinary circumstances, 

 a spectacle which has hitherto been neglected or imperfectly studied. I chose 

 for my point of observation a place called Haradowhill, two miles from Ames- 

 bury, and east of the avenue of Stonehenge, of which it closes the vista. In 

 front is that celebrated edifice upon which I knew that the eclipse would be 

 directed. I had, moreover, the advantage of a very extensive prospect in 

 every direction, being on the loftiest hill in the neighborhood, and that nearest 

 to the centre of the shadow. To the west, beyond Stonehenge, is another 

 rather steep hill, rising like the summit of a cone above the horizon. This is 

 Clay hill, adjoining Westminster, (?) and situated near the central line of dark- 

 ness which was to set out from this point, so that I could be aware in time of 

 its approach. I had with me Abraham Soirges and Stephen Evans, both na- 

 tives of the country, and able men. The sky, though overcast, gave out some 

 straggling rays of the sun, that enabled me to see around us. My two com- 

 panions looked through the blackened glasses, while I made some .reconnais- 

 sance of the country. It was half-past five by my watch when they informed 

 me that the eclipse was begun. We watched its progress, therefore, with the 

 naked eye, as the clouds performed for us the service of colored glasses. At ( 

 the moment when the sun was half obscured, a very evident circular rainl-ow ) 



1 **^s*^ 



