whichtvill happen on Septemler 1 , 1820. 99 



circumference and ended almost perpendicularly like a precipice. 

 My Lord Morton was able to see them very easily through his 

 smnll reflector." 



The king of France, who (as already mentioned) went purpose- 

 ly to Compiegne to observe this eclipse, discovered towards the 

 middle of the eclipse (which was not more than 9f digits) " sur 

 la surface de la lune, comprise entre les cornes du soleil, des 

 rayons de lumiere rouges, et un filet de lumiere qui sembloit 

 masquer le disque de la lune, et qui s'etendoit a une distance 

 des cornes*." 



M. De L'isle, in his publication above alluded to, seems to 

 think that a quick eye, guarded with a sufficiently dark glass, 

 might in solar eclipses discover the body or limb of the moon 

 seven or eight minutes before it touched the sun, and also for 

 the same time after it had left it and was entirely off the sun. 

 He remarks that the observer should defend himself as much as 

 possible from the direct light of the sun, and also from the light 

 of the external air. No person however has hitherto noticed such 

 an appearance : although many observers attended particularly 

 to it, in the eclipse of 1748, in consequence of M. De L'isle's 

 remarks. Should the moon in such case ever be visible, it would 

 enable us to determine with greater accuracy the commencement 

 of any solar eclipse f. 



During the progress of the eclipse it would be desirable to as- 

 certain the degree of cold and obscurity caused by the diminu- 

 tion of the sun's ravs : for which purpose, preparations should 

 be made beforehand, in order that no time be lost during the pe- 

 riod of the eclipse. The variations in the thermometer and ba- 

 rometer may be easily noted down without interrupting the astro- 

 nomical observations. The rapid change in the temperature of 

 the air may cause a hurricane of wind, (together with rain or 

 snow) as was observed about the middle of the eclips«i by Mr. 

 Maclaurin in 1737; and by Le Monnier in 1748. Mr. Short 

 says that (in the eclipse of 1748) " we did not at all perceive or 

 feel any greater degree of cold, during the eclipse, than v.e folt 

 before it began." But M. Cassini De Thury, who went with tlie 

 king of France to Compiegne to observe this eclipse, and where 

 it was only 9{- digits, savs they experienced a considerable de- 

 gree of cold at the time of the middle of the eclipse; the thcr- 



* Mem. (le l' Avail. Roy. des Sciences, 1748, page 56. 



f It must be evident to the practical astronomer, tliat if the moon were 

 I eally vioible in such cases, she would also be frequently visible at the con- 

 junctions, when no eclipse took place. M. De L'isle's sui^geslion arose 

 from a remark made by .M. Cassini on a luminous ring which was seen to 

 surround Mercury in its passage; across the sun's disc in the year \7M, anil 

 which continued for C) or 7 seconds after Mercury was entirely olV the sun's 

 disc. — iice Mem. dc I' /lead, des Science.^ for 173G, page.'J7>''. 



G 2 niomcter 



