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4 PREFACE. 



occiiltations that can be made available for this purpose on any part of the 

 American continent included between the parallels of 15° and 50° north. 



The selection comprises the larger planets and all stars to the fourth 

 magnitude, inclusive, which can be occulted to any part of North America 

 within the above-named limits, whether by day or night. In the observation 

 of an occultation taking place while the sun is above the horizon, the 

 observer must of course be guided in his search for the star by the moon 

 itself. If the points on the moon's border where the immersion and emer- 

 sion are to take place have been computed, there can be no difficulty, pro- 

 vided the telescope employed be of sufficient optical power to show the star 

 distinctly when once brought into the field. A very good observation may 

 be made, by daylight, with a three or a three-and-a-half feet telescope, if the 

 occultation be one of the largest class of stars; but, so far as my own expe- 

 rience goes, I do not think that a good observation can be obtained under 

 these circumstances, where the occultation is of a star less than that of the 

 second or third magnitude. An approximate knowledge of the longitude, 

 however, is always desirable where a more perfect one cannot be had, and it 

 was therefore thought advisable to extend the daylight occultations to stars 

 of the fourth magnitude. 



All the stars of the British Association Catalogue., of whatever magnitude, 

 that can be occulted in the absence of sunlight, between the times of change 

 and of full moon, and between the times of last quarter and of change, have 

 been selected from that work. For a few days following the full, the occul- 

 tations of stars smaller than of the sixth magnitude have been omitted ; the 

 difficulty of ascertaining the precise instant of their emersions being so great 

 at this period of the lunation as to render the observations useless. 



The general list has been still farther extended by including such stars 

 of the eighth and ninth magnitudes as can be occulted during the most 

 favourable period of each lunation, the first seven or eight days following 

 the change. All those of Lalande^s Catalogue of nearly fifty thousand stars, 

 and of Weisse's Catalogue of nearly thirty thousand stars, which can be occulted 

 under circumstances favourable for observation, are probably included in 

 this list. In the occultation lists published in Europe, stars of this class 

 are not included. I know of no reason why they should be rejected, unless 

 it be that computers shrink from the labour of making the necessary calcu- 

 lations. Though less interesting, perhaps, regarded merely as phenomena, 



