252 Mr. Baily on an [Oct, 



with sufficient accuracy by the assistance of a telescope magni- 

 fying 30 or 40 times ; together with a well regulated clock, or 

 watch, that beats seconds ; and which, if possible, should, as 

 before observed, be set to mean time on the day of the eclipse.* 

 I must again repeat, however, that the neglect of this precaution 

 should not deter the observer from noting down the duration of 

 the several phases above-mentioned, and particularly the times 

 at which the annuius is formed and dissolved : which may be 

 afterwards compared with more exact observations, and lead to a 

 correction of the true times.f 



It is presumed that the observer will also, from time to time, 

 during the progress of the eclipse, observe and note down the 

 distance and inclination of the cusps in the usual manner.'! It 

 may likewise be proper to remark that it will be of considerable 

 importance to ascertain, at the time of the middle of the eclipse, 

 the magnitude of the annuius on the north and on the south side 

 of the moon, in order to determine how far distant, at that time, 

 the centre of the moon is from the centre of the sun.§ If at the 

 same moment the observer can determine the diameter of the sun 

 and moon, it will add considerably to the importance of the 

 observation ; and tend to determine a much disputed point in 

 practical astronomy. || These observations, however, should be 

 made with a good telescope furnished with an accurate micro- 

 meter : and, in making a report thereof, the observer should 

 describe the kind of telescope made use of, as well as the method 

 employed in determining the magnitude of the annuius, &.c. For 

 the sake of greater accuracy, he should also make a diagram of 

 the appearance of the sun and moon, at the time of the middle of 

 the eclipse ; placing a mark against that part of the sun's disc 

 which appears the most vertical to him. The point on the cir- 



* In the evening of (he same day on which this eclipse takes place there will be 

 an eclipse of the first satellite of Jupiter : the immersion will take place at Green- 

 wich at Sh 34' 34" mean time. Tho^e persons, therefore, who are furnished with 

 sufficiently powerful telescopes, may (if the weather prove favourable) have an 

 opportunity of ascertaining the correctne-s of their clocks or watches. 



+ M. De L'lsle states, that if we observe the situation of the cusps, or only their 

 distance, at the time of the middle of the eclipse (when the eclipse is not annular, 

 hut nearly so), it will serve to determine (he apparent route of the penumbra and 

 its lim'ts, <x- exactly as if we had observed the duration of the annular eclipse. 



X There are two modes of observingthe phases of an eclipse of thi' sun : the one, 

 by looking directly at the sun, with a telescope furnished with a micrometer ; the 

 other, by receiving the image of the sun, through a telescope, on a screen, in a dark 

 chamber, or camera obscura. Fach has its advantages, and may be practised accord- 

 ing to circumstances. See Laltinde's Astronomie, vol. ii. p. 659. M. l)e L'lsle 

 indeed says, that " we may deiermirie, with sufficient exactness, the situation of 

 the cusps, without making use either of the d;irk chamber or the micrometer, by 

 observing the moment of the passage of the cusps and of the limbs of the sun, by 

 means of simple wires placed in the focus of the telescope, in any situation what- 

 ever; and leaving the telescope in a fixed position, during the time that the sun 

 employs to traverse the field of it." 



(j M. De L'lsle doubts whether this part of the observation can be made with 

 sufficient accuracy in a dark chamber; on account of the indistinctness of the 

 image of the moon. 



j) See next page. 



