which will happen on September 7, 1820. 95 



would not diacourage 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 depend- 

 ance can be placed on those observations which are not 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 telescopeSj &c. were invented. 



Those persons, however, who have the proper instruments, and 

 every conveniency for observing, will of course note down the 

 usual circumsta:ices in such case : viz. 



1. The time of the commencement of the eclipse* j 



2. The time of the formation of the annulus ; 



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



4. The time of the end of the eclipse. 



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

 vious 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 

 with sufficient accuracy by the assistance of a telescope magni- 

 fying thirty or forty times; together with a well regulated clock 

 Of watch tliat beats seconds; and which, if possible, should as 



may be (that is, the greater the number of placen where they are made), the 

 more important will be the consequences to be derived from them. Conse- 

 quently erery observation will be material. Those persons who may obsen-e 

 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 tele- 

 scope shouUl take in at least one-half of the circumference of the sun's disc 

 (taking that portion of it, which may leave the e.xpected 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 di.^c) 

 may be attended with advantage, as in the case of the solar eclipse on Sep- 

 tember .0, XyX^, where Sir W'm. Herschel observed that the first impression 

 on the sun's disc was made by the projection of two high mountains of the 

 moon, having the appearance of horus ; which were distinctly visible on the 

 .sun's disc before the body of the moon appeared. — I'hil. Trans. IJi'^, p. .'1.0. 



t In order to determine, with greater accuracy, the formation and disso- 

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

 only of the di.nc of the sun whicli is necessary for the purpose. By adopting 

 this method Mr. Maclaurin, in 17''i7. was enabled to observe the appearance 

 alluded to in page !W ; and which pre':cdcd the perfect formation of the an- 

 nulus about twenty seconds : thereby enabling him to look out (ox and note 

 down tilt exact time, with greater precision. 



before 



