of the Total Eclipse of July 28, 1851. 163 



totality each of the observers should repeatedly examine th« 

 whole circumference of the moon, to discover whether there 

 are any of these prominences visible. The observer with 

 the most powerful telescope should devote himself entirely 

 to this subject. If any are seen, it is of the utmost import- 

 ance to note whether they undergo any change ; whether 

 new ones appear, and in what part of the circumference ; 

 whether they increase on one side and diminish on the other, 

 &c. For details on this very important observation, see 

 Appendix No. II. The times of any striking phenomena 

 should be recorded, no description beyond reference by a 

 single word being attempted at the time ; and their places 

 should be noted on the card- circle. 



21. The telescope-observers should endeavour to judge 

 whether the disc of the moon is sensibly illuminated. Little 

 confidence can be placed in the appearance of light, unless 

 some of the larger spots can be seen. The sextant-observer 

 should measure the moon's diameter. If there is leisure, 

 one actinometer-observation should be made. 



22. An attempt should be made (as has already been 

 mentioned under article 14) to ascertain wliether the light 

 of the corona is sufficient to cast a distinguishable shadow, 

 and whether a distance can be found for the candle at which 

 the intensities of the shadows are sensibly equal. But it is 

 certain that the light of the corona is extremly feeble, and the 

 observer must therefore be prepared to remove the candle to 

 a considerable distance. Some estimate may be formed of 

 the intensity of light by remarking the distance at which the 

 letters and figures of a book can with difficulty be distin- 

 guished. All observers, as far as possible, should use the 

 same page ; for instance, the title-page of the Nautical 

 Almanac for 1851, or the title-page of these " suggestions," 

 in which the same type is used. To give this observation 

 its greatest value, each observer should as soon as possible 

 examine at what distance he can distinguish the same letters 

 in full sunshine, and at what stage of twilight and in what 

 position he finds the difficulty nearly the same as during the 

 eclipse. 



23. Should any stars or planets be seen, their places 



l2 



