1818.] Annular Eclipse of the Sun. 253 



cumference of the sun's disc (relative to its vertical or horizontal 

 diameter) where the moon leaves it in order to form the anmilus, 

 and again where it touches it at the time of the dissolution of 

 the annulus, should also, if possible, be distinctly noted. M. Le 

 Monnier considers this of considerable importance.* 



It was observed, in the annular eclipse of 1737, that the annu- 

 lus was formed and dissolved very suddenly. For when the 

 whole body of the moon had entered on the disc of the sun, the 

 last portion that entered appeared to adhere to the concave cir- 

 cumference of the sun's disc for some seconds ; and the moon 

 appeared elongated on that side, till the sun's light suddenly 

 broke round it, when the moon reassumed its regular curvature. 

 In a similar manner, when the disc of the moon approached the 

 concave line of the. sun's disc on the other side, they seemed to 

 run together like two contiguous drops of water on a table when 

 they touch each other. 



It was also observed, in the eclipse of 1737, " that as the 

 annulus was forming-, the light appeared to break in several irre- 

 gular spots near the point of contact : and that the limb of the 

 moon seemed to be indented there." These irregular parts 

 seemed likewise to have a kind of motion ; although there was no 

 undulation at the same time in the circumference of the sun. 

 ^uch appearances of a tremulous motion, in certain periods of 

 solar eclipses, are mentioned by Hevebus and others. It was 

 noticed also in the eclipse of 1748.+ 



In both these eclipses, as well as in that of 1764, it was 

 observed, that when the annulus was formed, the moon appeared 

 much smaller on the sun than it reallv ought to be ; and indeed 

 much smaller than the calculations seemed to warrant. But 

 whether this phenomenon arises from an apparent enlargement 

 of the sun's disc, or from an apparent diminution of the moon's 

 disc, or from both, does not seem clearly decided. M. Du 

 Sejoor i.as discussed this subject, with his usual ability, in his 

 Traite ana/t/tique des Mouvemens apparent des corps celestes, 

 vol. i. page 405, ice. ; but he has not come to any precise deter- 

 mination thereon. The observations have not been made with 

 sufficient accuracy, nor are they sufficiently numerous to enable 

 us to determine so nice an element in the calculation of eclipses. 



* In hi? paper Snf VVtUiU des Eclipses de Soleil wherein he has drawn many im- 

 portant consequence* from the eclipse of 17-iS) he remarks, respecting the method 

 of dfterminiiii the limits of the umbra, that " la phtpart des observateurs, ea 

 parol cas, -iment les routes ordinaire.-, et n'ont jamais fait assez d'attenlioa aa 

 point de lacirconfgrence dn limbe du soleil oik se forment les rapture? de I'anneau: 

 • fiiiai? cr, points de la cireonfjreoce du disque du soleil seront les plus import- 

 am, ei nous foarniront le- h.ii,-- que nous voudrens bien r.ssigner.'' — Mein.de 

 1'Aiad. if Bi i act a for 1165, p. -163. 



+ The Kev. Mr. Irwin, who noticed the erlipse of 17-18 at Elgin, say; thai "the 

 formation and breaking of the annnlns were sensibly to be observed, and passed In 

 O moment; affording a verv pleasing -ighl bv the irregular tremulous spots o. 

 MB."— Phil. Iran?, vol. x)v. p. 59j. 



