1818.] Annular Eclipse of the Sun. 255 



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 particu- 

 larly 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 com- 

 mencement of any solar eclipse.* 



During the progress of the eclipse it would be desirable to 

 ascertain the degree of cold and obscurity caused by the dimi- 

 nution of the sun's rays ; for which purpose preparations should 

 be made before-hand, in order that no time be lost during the 

 period of the eclipse. The variations in the thermometer and 

 barometer may be easily noted down without interrupting the 

 astronomical observations. The rapid change in the tempera- 

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

 or snow), as was observed about the middle of the eclipse by 

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

 Bays, that (in the eclipse of 1748) " we did not at all perceive 

 or feel any greater degree of cold, during the eclipse, than we 

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

 with the King of France to Compiegne to observe this eclipse, 

 and where it was only 9-i- digits, says, they experienced a con- 

 siderable degree of cold at the time of the middle of the eclipse ; 

 the thermometer, however, fell only 24-° : and the Abbe Nollet 

 found that his burning-glass was then as powerful as before the 

 eclipse began. M. De LTsle, likewise, who observed this 

 eclipse at the Luxembourg, remarks, that the thermometer did 

 not indicate any increase of cold caused by the eclipse, although 

 he and many other persons experienced it soon after the middle 

 of the eclipse. + 



In the eclipse of 1737, Maclaurin observed that a burning- 

 glass which kindled tinder and burned cloth towards the end of 

 the eclipse, had no effect during the existence of the annulus, 

 nor for some time before and after it. He likewise remarked,. 

 that " during the appearance of the annulus, the direct light of 



• It must be evident to the practical astronomer, that if the moon were really 

 visible in such cases, she would also be frequently visible at the conjunctions, 

 when no eclipse look place. M. De L'Isle's suggestion arose from a remark made 

 by M. Cassini on a luminous ring which was seen to surround Mercury in its pas- 

 tage across the suu's ditc in the year 1736, and which continued for six or seven 

 seconds after Mercury was entirely off the sun's disc. See Mem. de l'Acad. des 

 Sciences for 1736, p. 373. 



t In the total eclipse of 1724 the thermometer had fallen only two degrees at 

 the time of the middle of the eclipse. This is the more remarkable as the eclipse 

 took place late in the afternoon of May 22, at which time we might presume that 

 the atmosphere would be gradually becoming more cool. The total darkness took 

 ylace at 6*" 48' p. ■). 



