PREFACE. 5 



their occultations are equally valuable for the determination of longitudes 

 with those of the larger stars ; and, if we consider their much more frequent 

 occurrence, they give a very great advantage which the others do not possess, 

 and which ought not to be neglected. The observations are not difficult, 

 and do not, as many, suppose, require instruments of great power. A three 

 or three-and-a-half feet telescope is sufficient. JSTor is it necessary that the 

 daylight be entirely ended. An occultation, which deserves notice on account 

 of the smallness of the star and the early hour at which it took place, was 

 observed at the Friends' Observatory in Philadelphia on the 6th of June, 

 1848. The computed sidereal time of the end of twilight on that evening 

 was 14* 29"'. The star was No. 3418, British Association Catalogue, of the 

 eighth magnitude. The immersion took place at IS'' 29'" 21'.4, sidereal time, 

 or one hour before the end of twilight. The observations were made with a 

 seven feet equatorial and with a three-and-a-half feet portable telescope, and 

 were perfectly satisfactory with both instruments; the noted time of the 

 star's disappearance being almost identical. The star was readily found on 

 directing the small telescope to the adjacent limb of the moon, at least ten 

 minutes before its immersion, and was steadily and distinctly visible up to 

 the instant of occultation. From the perfect ease with which it could be 

 seen, there is no doubt that good observations could have been obtained, 

 had the star been of the ninth or even of the tenth magnitude. 



The Washington mean solar time, T, the Washington hour-angle of the 

 star, H, the log. sine and cosine of the star's declination, the co-ordinates 

 |j, q, and their hourly variations, ])', q', are given for the purpose of facili- 

 tating the computation of the occultations for any given place. This 

 arrangement leaves no further labour to be performed by the computer than 

 what depends on the longitude and latitude of his position. This labour is 

 by no means so serious as might be apprehended from a first glance at the 

 example worked out on pages 22 and 23. Every part of the process is there 

 given, and the work certainly appears of some length; but, with suitable 

 logarithmic tables, the calculations are very rapidly made, and an expert 

 computer will find it easy to determine all the circumstances necessary to 

 an observation in a few minutes. 



By the last column of the general list, will be seen on what part of 

 the continent an occultation, not visible at Washington, may be expected 

 to take place under favourable circumstances for observation. Where 



