1818.] Annular Eclipse of the Sun. "251 



person, to whom the opportunity may occur, will omit to note 

 down the particulars ; or fail to communicate the same to some 

 person conversant with the subject of astronomy. It will be of 

 equal importance to know that the existence of the annulus is 

 only momentary : or even that it is nearly, but not completely 

 formed.* 



Athough the possession of proper instruments must give a 

 superior degree of credit to the observations of any person ; yet 

 I would not discourage those who have not this advantage from 

 communicating any circumstances that may occur. For it has 

 been justly observed by M. De L'Isle, that although no great 

 dependance can be placed on those observations which are net 

 made with a telescope, &c. yet that such observations as are 

 made with the naked eye ought not to be entirely neglected ; 

 since it affords us an opportunity of judging of the accuracy of 

 those observations which were made before telescopes, &c. were 

 invented. 



Those persons, however, who have the proper instruments, 

 and every conveniency for observing, will of course note down 

 the usual circumstances in such case : viz. 



1. The time of the commencement of the eclipse.t 



2. The time of the formation of the annulus. 



3. The time of the dissolution of the annulus.J 



4. The time of the end of the eclipse. 



If there should be any spots on the sun, it will be proper 

 (previous to the commencement of the eclipse) to make a diagram 

 of the sun's disc ; and to note down the times when the body of 

 the moon comes in contact with the spots, and likewise the times 

 when they again become visible. All these may be determined 



» To those who are not much conversant with practical astronomy, it may, per- 

 haps, he proper to remark, that the more numerous these observations may be 

 (that is, the greater the number of places where they are made), the more import- 

 ant will be the consequences to be derived from them. Consequently (very obser- 

 vation will be material. Those persons who may observe the eclipse in the 

 country should state the distance and position of such place from the nearest 

 principal town. 



+ It is rather difficult to determine the exact time of the commencement of any 

 solar or lunar eclipse; since the impression on the disc does not become visible till 

 some seconds after the eclipse has begun. The field of the telescope should take in at 

 lea>t one half of the circumference of the sun's disc (taking that portion of it which 

 may leave the expected point of contact in the centre), as the eye can much better 

 judge of any impression made upon a large, than a small portion of a circle. In 

 some cases, however, a very powerful telescope (which takes in only a small portion 

 of the sun's disc) may be attended with advantage, as in the case of the solar eel ipse 

 on Sept. 5, 1793, where Sir Wm. Herschel observed that the first impression on tlie 

 sun's disc was made by the projection of two high mountains of the moon, having 

 (be appearance of horns; which were distinctly visible on the sun's disc before tha 

 body of the moon appeared. — Phil. Trans. 1794, page 39. 



J In order to determine, with greater accuracy, the formalion and dissolution 

 of the annulus, the observer should take into his telescope that part only of the disc 

 of the sun which is necessary for the purpose. By adopting this method Mr. 

 Maclaurin, in 1737, was enabled to observe the uppearance alluded to in page 254 ; 

 and which preceded the perfect formation of the annulusabout 20 seconds; thereby 

 enabling him to look out for and note down the exact time with greater precision. 



